Thank you ldonahue for your reply.
I went through that tutorial, and created a simple add-in with just one button popping up a message. The add-in installed ok (I got the "Installation succeeded" message, and the add-in is listed in the Add-in manager), but when I go to the Commands list to add the button to an existing toolbar, the "Java Commands" category (where the button is supposed to be) isn't even there.
I then checked my system's default JRE version, which actually was 64 bit. I removed it from the system, so now the default version is the 32 bit 1.7.0_02, but my problems are still here...
Any other suggestion?
Alberto