I have the same problem with iPhoto 9.3's advertised ability to import AVCHD videos, and I also carefully followed Apple's instructions. (I am using the latest version of Mac OS X too; 10.7.4.) I get lots of "unreadable files" due to "unrecognized format", even with MTS files.
In addition, Apple's instructions imply that the camera should either mount and allow AVCHD movies to be imported in the usual way, or not mount at all, in which case Apple's instructions for copying appropriate folders should be followed. However, my Panasonic FZ150 mounted normally, but no AVCHD movies showed in the list to be imported.
I reported this to Apple via "Provide iPhoto Feedback" in the iPhoto menu.
Has anyone got this to work with the publicly-released version of iPhoto 9.3?