A basic example / sanity check for setting up and using JBullet. This follows the demos fairly closely. The source for them is available here:
Left click to apply a force, right click to reset.
Download it here: http://jbullet.advel.cz/
Create a folder in your Processing libraries folder called jbullet and a folder inside that called library and copy jbullet.jar from the zip to that location. Do the same for vecmath and vecmath.jar from the zip. Import both into your sketch. I believe that covers it but I could be wrong.