Hi,
Coincidentally, I did the exact same thing as you did yesterday.
MC basically has no link to- or knowledge of Airfoil directly... it is in Airfoil that you select MC 19 as audio source, and you select in Airfoil to which Airplay devices the music is streamed.
Add to this Airfoil Remote and JRemote on iPad, and you have a pretty much ideal setup.
I first wanted to use DLNA to stream audio to different locations in the house (with MC on PC), but there were always some devices that didn't work as expected.
Hence the move to using Airplay. I first tested Airfoil on PC, but the volume was not as loud + Airfoil Remote is only available for Airfoil on OSX at the moment.
So I just bit the bullet and bought MC 19 for Mac, Airfoil for Mac and Airfoil Remote on iPad (I already had JRemote).
It is true that you can select an airplay device directly in OSX, but it is not possible to stream to multiple zones simultaneously, and changing airplay devices is buried in the system preferences, which is not very user-friendly, nor can you change Airplay device remotely (iPad, phone, ...).
It would be perfect if one could create a different MC zone for each airplay device, but that is unfortunately not possible.
Good luck with your setup! You will be pleased when all is working :-)
Cheers,
Charles