For my system, I have apple airport express and Apple TV. For Apple TV, there is one optical digital output. Providing that the Apple TV is connected within the same wifi network. The optical digital output is connected to the DAC's Optical (toslink) digital input, and select the Optical digital input on the DAC. The DAC should also be connected to the audio system. Then the user can use the Apple TV's remote to play the music that are shared with the same network's (Home Sharing). Also, make sure the iTunes on the computer has to be opened. For more details, Apple website should have step by step set up. It is not hard at all to follow.
For Apple airport express, the user has to program it to share the iTunes music first. Again follow the setup procedures on Apple website.(or airport utilities). Once it is set up properly, Connect the Optical digital output on the Apple airport express to the DAC. To control the playback, just launch Itunes on the computer and click to choose Sharing (Apple TV/?? forgot what that call..hehe), That's about it!
Again, Check the detailed procedures on Apple website. Or if possible (still not sure what to do, make the appointment at IFC apple store, they should be able to assist!
Actually, It is not hard at all. Just try first....
BTW, the Design purpose of Airplay is to let the users to share music within a house/home easily without worrying to connect the cables. Just enjoy listening to music whenever and wherever the users want (again within the same wifi network). But of course, the quality of the sound may not be top rated.