I was the Author of that part of LinuxMCE. I did not yet have time to write a wiki page, it's on the ToDo List.
Basically, you enable it for the MD's you want to appear as AirPlay target's. Once you have enabled it for at least one Media Director, you should have an AirPlay Plugin device on your Core, and an AirPlay Player device on the MD. Don't forget to quick reload.
Once you did that the MD should show up as AirPlay target on your iOS device, but attention: At the moment it's only for Music Playback and not yet for video or image playback ...