From your question it is not clear whether you are trying to find a file that is on the BlackBerry or one that you created in your JDE project. If it is on the BlackBerry file system then this should work ok - you'd want to make sure that the file test.txt is really in the user directory. I haven't done this yet myself but I was able to read a new picture taken by the camera using the line
String filePath = "file:///SDCard/BlackBerry/pictures/test.jpg";
FileConnection fc = (FileConnection)Connector.open(filePath);
I'm still just using the 9000 simulator and the eclipse plug-in.
The actual photo when I select it and look at the Properties claims it is
at /Media Card/BlackBerry/pictures/test.jpg however.
If you are trying to access a file which you created in you project folder then see the post on BlackBerry's - Java Development - BlackBerry Support Community Forums titled (google blackberry and this title):
"Not able to read text or binary file"