If you've ever tried to import or AMA link AVCHD media on Mountain Lion you'll know what I'm talking about. For those who don't, essentially Apple want us to open AVCHD using QuickTime player. Fine. However, in their wisdom they've disabled access to the .MTS files, and instead the 'PRIVATE' folder appears to the OS as a QuickTime file.
The side effect is that Media Composer has no way of getting into the folder containing the .MTS files, so it can't import or AMA them.
Here's the solution until Apple pull their collective fingers out...
1. Create a new folder anywhere on your computer. Doesn't matter what it's called. I called mine MTS.
2. Inside the AVCHD backup right-click on the PRIVATE folder and select 'Show package contents'
3. Inside there's a folder called AVCHD. Right-click and select 'Show package contents' again.
4. Do the same with BDMV
5. Inside the folder called STREAM is where the .MTS files are kept. Select all, then right-click and select 'Make Alias' (this creates a shortcut to each of the files)
6. Drag all of the new alias files into your folder from step 1.
Avid will be able to read the alias files correctly via AMA and Import. The benefit here is that alias files are just links to the proper MTS files, so you're not creating any new media in the process.