JHotDraw = Trouble

I'm trying to use JHotDraw to create a diagram tool but it seems that I can't get JHotDraw to work!! When I try to start the JModeller that I use as an inspiration (!!) it starts OK, but when I try to add a new class figure, the program crashes with a NullPointerException, and a java.lang.classNotFoundException on the ClassFigure.java file!!!

I'm wondering if anyone else is, or have, experienced theese kind of problems and perhaps how to solve them!!

The guy i work with doesn't have any problems so it's weird that I can't get it work!! We are doing the exact same thing... Any ideas to solve it?!

Thanks in advance...

JHotDraw sounds like an extension - are you sure you set it up correctly?

Yes JHotDraw is an extension for Java, its an open Source Framework for creating programs for diagram drawing and more...

About the set up... I'm not quite sure that there really is any set up to do... The only thing you have to do is tell your compiler to use the .jar file... And my IDE(programming program:) ) is doing this by itself... Otherwise I wouldn't be able to even compile my Java files, and that is going smooth!!
The problem occurs when I make a class that extends from GraphicalCompositeFigure (or use any other class that does this)! Why?! I don't know, but I would like to know if any one haved had this problem and perhaps knows how to solve it...