For anyone like me who is searching how to actually install 3rd party bundles into TM2, getting confused by the older posts here mentioning Avian folders and what not, here's my micro-summary how it currently worked for me, pretty easy, using Gerd's MultiMarkdown as an example:
1. Go to ~/Library/Application\ Support/TextMate/Managed/Bundles
2. git clone git://github.com/gknops/MultiMarkdown.tmbundle.git This creates the MultiMarkdown.tmbundle folder
3. Restart TM2 That's right, no cache deleting required, the MultiMarkdown bundle appeared both in the Bundle manager and in the list of installed bundles under Menu->Bundles. Also, the multimarkdown command (that I installed with homebrew) was automatically found and used by the bundle.
As often, this turned out to be easier than I thought. ;)
Thx everybody,
Michael
PS.: Using TM2.0-alpha.9387 on 10.8.2