this is how to create a file selection popup screen

/*
 * FileSelectorPopupSample.java
 *
 * © Research In Motion, 2003-2009
 * Confidential and proprietary.
 */

package com.samples.fileSelectorPopup;

import net.rim.device.api.ui.UiApplication;
import net.rim.device.api.ui.component.Dialog;
import net.rim.device.api.ui.container.MainScreen;
import net.rim.device.api.ui.MenuItem;
import java.lang.String;

/**
 * A sample application demonstrating the use of a custom
 * FileSelectorPopupScreen.
 */

public final class FileSelectorPopupSample extends UiApplication
{
    public static void main(String[] args)
    {
        FileSelectorPopupSample theApp = new FileSelectorPopupSample();
        theApp.enterEventDispatcher();
    }

    public FileSelectorPopupSample()
    {
        MainScreen mainScreen = new MainScreen();
        mainScreen.setTitle("FileSelectorPopupScreen Example");

        MenuItem showAllMenu = new MenuItem("Select a File", 40, 40)
        {
            public void run()
            {
                FileSelectorPopupScreen fps = new FileSelectorPopupScreen();
                fps.pickFile();
                String theFile = fps.getFile();

                if (theFile == null)
                {
                    Dialog.alert("Screen was dismissed. No file was selected.");
                }
                else
                {
                Dialog.alert("File selected: " + theFile);
                }
            }
        };

        mainScreen.addMenuItem(showAllMenu);

        MenuItem showImagesMenu = new MenuItem("Select an Image", 40, 40)
        {
            public void run()
            {
                String imageExtensions[] = {"jpg", "jpeg",
                     "bmp", "png", "gif"};

                FileSelectorPopupScreen fps = new FileSelectorPopupScreen(null, imageExtensions);
                fps.pickFile();
                String theFile = fps.getFile();

                if (theFile == null)
                {
                    Dialog.alert("Screen was dismissed. No file was selected.");
                }
                else
                {
                    Dialog.alert("File selected: " + theFile);
                }
            }
        };

        mainScreen.addMenuItem(showImagesMenu);

        pushScreen(mainScreen);
    }
}
/*
 * FileSelectorPopupScreen.java
 *
 * © Research In Motion, 2003-2009
 * Confidential and proprietary.
 */

package com.samples.fileSelectorPopup;

import net.rim.device.api.ui.container.PopupScreen;
import net.rim.device.api.ui.UiApplication;
import net.rim.device.api.system.Bitmap;
import net.rim.device.api.system.Characters;
import net.rim.device.api.ui.component.*;
import net.rim.device.api.ui.container.DialogFieldManager;
import javax.microedition.io.Connector;
import javax.microedition.io.file.*;
import java.util.*;

/**
 * A PopupScreen with a file browser allowing for file selection.
 */

public class FileSelectorPopupScreen extends PopupScreen
{

    String _currentPath; //The current path;
    String[] _extensions; //File extensions to filter by.
    ObjectListField _olf; //Lists fields and directories.

    /**
     * Open the screen to the root folder and show all files
     * and directories.
     */
    public FileSelectorPopupScreen()
    {
        this(null, null);
    }

    /**
    * Open the screen starting at the specified path and filter
    * results based on a list of extensions.
    * @param startPath The initial path to open.
    * Use null to start at the root file systems.
    * @param extensions Allowable file extensions to display.
    * Use null to display all file types.
    */
    public FileSelectorPopupScreen(String startPath, String[] extensions)
    {
        super(new DialogFieldManager());
        _extensions = extensions;
        prepScreen(startPath);
    }

    /**
     * Display the screen, prompting the user to pick a file.
     */
    public void pickFile()
    {
        UiiApplication.getUiApplication().pushModalScreen(this);
    }

    /**
     * Retrieves the current directory if the user is still
     * browsing for a file, the selected file if the user has      * chosen one or null if the user dismissed the screen.
     * @return the current directory if the user is still
     * browsing for a file, the selected file if the user has
     * chosen one or null if the user dismissed the screen.
     */
    public String getFile()
    {
        return _currentPath;
    }

    //Prepare the DialogFieldManager.
    private void prepScreen(String path)
    {
        DialogFieldManager dfm = (DialogFieldManager)getDelegate();
        dfm.setIcon(new BitmapField(Bitmap.getPredefinedBitmap(Bitmap.QUESTION)));
        dfm.setMessage(new RichTextField("Select a file"));

        _olf = new ObjectListField();
        dfm.addCustomField(_olf);

        updateList(path);
    }

    //Reads all of the files and directories in a given path.
    private Vector readFiles(String path)
    {
        Enumeration fileEnum;
        Vector filesVector = new Vector();

        _currentPath = path;

        if (path == null)
        {
            //Read the file system roots.
            fileEnum = FileSystemRegistry.listRoots();

            while (fileEnum.hasMoreElements())
            {
                filesVector.addElement((Object)fileEnum.nextElement());
            }
        }
        else
        {
            //Read the files and directories for the current path.
            try
            {
                FileConnection fc = (FileConnection)Connector.open("file:///" + path);
                fileEnum = fc.list();
                String currentFile;

                while (fileEnum.hasMoreElements())
                {
                    //Use the file extension filter, if there is one.
                    if (_extensions == null)
                    {

                        filesVector.addElement((Object)fileEnum.nextElement());
                    }
                    else
                    {
                        currentFile = ((String)fileEnum.nextElement());

                        if (currentFile.lastIndexOf('/') ==
                           (currentFile.length() - 1))
                           {
                           //Add all directories.
                           filesVector.addElement((Object)currentFile);
                           }
                        else
                        {
                            //This is a file. Check if its
                            //extension matches the filter.
                            for (int count = _extensions.length - 1;
                                  count >= 0; --count)
                            {
                                if (currentFile.indexOf(_extensions[count]) != -1)
                                 {
                                      //There was a match, add the file and
                                      //stop looping.

                                      filesVector.addElement((Object)
                                          currentFile);
                                      break;
                                 }
                            }
                       }
                  }
                }
             }
              catch (Exception ex)
              {
                  Dialog.alert("Unable to open folder. " + ex.toString());
              }
        }
        return filesVector;
    }

    //Handles a user picking an entry in the ObjectListField.
    private void doSelection()
    {
        //Determine the current path.
        String thePath = buildPath();

        if (thePath == null)
        {
            //Only update the screen if a directory was selected.
            updateList(thePath);
        }
        else if (!thePath.equals("*?*"))
        {
            //Only update the screen if a directory was selected.
            //A second check is required here to avoid
            //a NullPointerException.
            updateList(thePath);
        }
        else
        {
            //The user has selected a file.
            //Close the screen.
            this.close();
        }
    }
    //Updates the entries in the ObjectListField.
    private void updateList(String path)
    {
        //Read all files and directories in the path.
        Vector fileList = readFiles(path);

       //Create an array from the Vector.
       Object fileArray[] = vectorToArray(fileList);

       //Update the field with the new files.
       _olf.set(fileArray);
    }

    //Build a String that contains the full path of the user's selection.
    //If a file has been selected, close this screen.
    //Returns *?* if the user has selected a file.
    private String buildPath()
    {

        String newPath = (String)_olf.get(_olf, _olf.getSelectedIndex());

        if (newPath.equals(".."))
        {
            //Go up a directory.
            //Remove the trailing '/';
            newPath = _currentPath.substring(0, _currentPath.length() - 2);

            //Remove everything after the last '/' (the current directory).
            //If a '/' is not found, the user is opening the
            //file system roots.
            //Return null to cause the screen to display the
            //file system roots.
            int lastSlash = newPath.lastIndexOf('/');

            if (lastSlash == -1)
            {
                newPath = null;
            }
            else
            {
                newPath = newPath.substring(0, lastSlash + 1);
            }
        }
        else if (newPath.lastIndexOf('/') == (newPath.length() - 1))
        {
            //If the path ends with /, a directory was selected.
            //Prefix the _currentPath if it is not null (not in the
            //root directory).
            if (_currentPath != null)
            {
                newPath = _currentPath + newPath;
            }
        }
        else
        {
            //A file was selected.
            _currentPath += newPath;

            //Return *?* to stop the screen update process.
            newPath = "*?*";
        }

        return newPath;
    }

    //Saves the files and directories listed in vector format
    //into an object array.
    private Object[] vectorToArray(Vector filesVector)
    {
        int filesCount = filesVector.size();
        int dotIncrementor;
        Object[] files;

        //If not in the root, add ".." to the top of the array.
        if (_currentPath == null)
        {
            dotIncrementor = 0;
            files = new Object[(filesCount)];
        }
        else
        {
            dotIncrementor = 1;
            files = new Object[(filesCount + dotIncrementor)];

            //Add .. at the top to go back a directory.
            files[0] = (Object)("..");
        }

        for (int count = 0; count < filesCount; ++count)
        {
            files[count + dotIncrementor] =
               (Object)filesVector.elementAt(count);
        }

        return files;
    }

    //Handle trackball clicks.
    protected boolean navigationClick(int status, int time)
    {
        doSelection();
        return true;
    }

    protected boolean keyChar(char c, int status, int time)
    {
        //Close this screen if escape is selected.
        if (c == Characters.ESCAPE)
        {
            _currentPath = null;
            this.close();
            return true;
        }
        else if (c == Characters.ENTER)
        {
            doSelection();
            return true;
        }

        return super.keyChar(c, status, time);
    }
}

[1] http://www.blackberry.com/knowledgecenterpublic/livelink.exe/fetch/2000/348583/800332/800505/800256/How_To_-_Create_a_File_Selection_Popup_Screen.html?nodeid=1498743&vernum=0