We are about to switch to a new forum software. Until then we have removed the registration on this forum.
The latest version of ruby-processing includes example usage of jbox2d using the pbox2d gem. For a full list of included samples see the examples on github. To read more about ruby-processing see wiki.
PS if you have previously installed sample sketches, please remove/move them before installing the new examples (the installer is not clever enough to warn you).