To set it up, change the Port 1 controller to "Steering Wheel" under the GameCube tab. This will tell the game that you have a force feedback steering wheel connected.
In the Gamecube Pad Settings, change the Rumble Motor to "Constant".
Configure the controls:
Main Stick Left/Right = Steer Left/Right
Main Stick Up = Accelerate
Main Stick Down = Brake
Thanks to ulao for the device communications info.