I recently bought a new MBP with Snow Leopard (10.6.4), restored from my Time Machine backup, and installed the latest MaxMSP (5.1.5). I started having many issues, one of which was the audiounit~ object wouldn’t work.
I noticed that logging out of my usual account into a fresh user account fixed a bunch of issues, which often points to stored preferences.
Solution: trash the ~/Library/Preferences/Max5 Preferences Folder. Everything works great now.