Hi moritz,
I just saw your post. 'app' was a variable I was using to set the root PApplet in my application. for your example, just change it to 'this' and it should work.
The class, or more specifically, GSVideo needs a PApplet to send it's movieEvent to, so I pass the main PApplet into the class then it can pass it along.
Hope this helps, Good luck
ak