The StagesBike can serve as power meter, cadence sensor, and controllable trainer within Zwift. Simply select StagesBike as your power source, then select it for Cadence and Controllable. 

Note: From the power source pairing, you will also see the bike's power meter (both left and right if you are searching via Bluetooth). There is no need to pair to the power meter itself as the power meter is sending data directly to the bike.