If you want, I can try to help you. However, while searching the Parallax message boards for 2.5 syntax documentation, I found that somebody already wrote a MacOS X native IDE. It still requires the keyspan adapter, but VirtualPC is out. Anyway, the author's site is
http://www.interaction.rca.ac.uk/alu.../murat/MacBS2/. I don't think it's open-source, but it's free-of-charge.