I do not have AD2 so the only advice I can give is to make sure you've done all of your updates first. You now have an Instrument Plug-ins drop down Menu inside the latest version of Garageband. Reading over the Support Page for AD2 Garageband is not officially listed.
From XLN Audio Website:
We officially currently support the following hosts (meaning we continually test and make sure our
software works):
- Cubase 5, 6, 7
- Pro Tools 10, 11
- Live 8, 9
- Logic Pro 9, X
- Sonar X2, X3
- Reaper
NOTE: Although we don’t officially support other hosts, our products usually work just fine. You can download demo versions of Addictive Drums 2 and Addictive Keys to make sure everything works as expected.
Pict - Addictive Drums with Garageband.

