Updating to OS X (10.6.4) and Max (5.1.5) using Time Machine restore
I posted this over in the Edge forum as well.
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.
Hope this helps someone in the future.