With AirPlay, you can't: Apple has recently (see Airfoil Speakers Touch - story at http://daringfireball.net/2012/05/more_on_airfoil_speakers_touch ) removed all the audio AirPlay receivers from the AppStore.
HOWEVER! There are a plethora of third-party audio servers capable of sending audio to iPads. For example, you can easily start a radio server on your desktop, which most radio receiver apps will be able to play back. Also, there are a lot of specialized p2p audio streaming apps.
Going this way, unfortunately, requires you to install a third-party server on your Mac.