To connect the Air to your network via Ethernet, disconnect the power cable from the rear of the Air and connect an Ethernet cable between the router and the Air. An Ethernet cable should be included in the box with your Air.
Next, reconnect the power cable to your Air and press the power button to turn on the unit.
After several seconds, the WPS light on the rear of the Air should illuminate with a solid green colour - this indicates that a wired network connection has been established, and the Air should be ready to use with Airplay and internet radio.