Just as Java's CLASSPATH must contain the correct hierarchy of directories and archives to access the required packages and classes, the point at which you mount new file systems in the IDE Filesystems is very important.

Make sure you mount a directory that contains the default package. For example, if you have classes in package com.yoyodyne.spreadsheet, and they are contained in directory C:\JavaDevel\source\com\yoyodyne\spreadsheet\, be sure to mount the directory C:\JavaDevel\source\.

Similarly, when using JAR files, be sure that Java packages match the position of classes in the JAR. Java requires you to do this too, so it is a good habit.