What do I need to mount a rim?

To use a wheel rim, you will need a Wheel Hub, such as the ClubSport Universal Hub or Podium Hub. 
The wheel side QR (Quick Release) + the base side QR will connect your Wheel Rim / Hub combo to the Wheel Base.

Optionally, you can also add a Button Module and a Shifter Paddle Module for enhanced functionality. For a complete setup, pedals are also required.
Note that rims cannot be directly mounted onto a Quick Release; a Hub is necessary to connect the Rim to the Quick Release Wheel Side.

This schematic may help to understand the world of modular steering wheels @ Fanatec