Third-party software instruments with Mainstage 2.2

The Mainstage 2.2 user manual and the product description on the AppStore both say that third-party software instruments can be used with Mainstage 2.2. I can't figure out how to do this, even though all my third party AU plugins have been verified in Mainstage as usable. When I try to add a software instrument via the channel strip, only Logic and Garageband instruments appear.

Mac OS 10.6.8

Mainstage 2.2