Here are the steps
1) First step is look at your System Preferences to see if your computer has any available AirPlay devices it can send audio to. An often overlooked fact is thatGarageBand can only use input and output devices that the OSX recognizes.
2) No Go to GarageBand Preferences and select the Audio?MIDI tap
3) From the Audio Output popup menu, you should see the Airplay option. Select it.

A Tip:
If you want to use Aiplay, I would suggest that you setup the "Multi Output". THis lets you send audio to multiple destinations at the same time.
If you don't see those option, it might be that you are using Lion (10.7) and not Mountain Lion (10.8), I'm not sure when those features were implemented. Maybe a reason to upgrade.
Hope that helps
Edgar Rothermich
http://DingDingMusic.com/Manuals
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'