To enter the key, hold down the Option key then choose your network from the AirPort menu. You should now be able to enter the pre-shared WPA key.
AirPort: How to enter a hexadecimal 64-character pre-shared key for WPA
Networks using Wi-Fi Protected Access (WPA) normally require a plain-text password or passphrase. Some networks may require you to enter a hexadecimal 64-character key instead.