We recommend having a quick look at the Airfoil Speakers manual, always accessible from the Airfoil Speakers menu.

Using Airfoil Speakers
Airfoil Speakers enables you to stream audio to other Macs on your local network, having them act just like an AirPort Express. Just run Airfoil Speakers on any Mac to which you wish to send audio, and it will appear in Airfoil's main window.

Click the Transmit button next to the desired remote speakers in Airfoil, and Airfoil Speakers will receive the audio.