Preliminary notes:
Sterzo Smart and Rizer are accessories that can be only paired on Zwift via Bluetooth Smart connection. Check if your device supports the Bluetooth Smart protocol.
The Rizer interfaces directly with the trainer to control the bike inclination because it is actually the trainer that communicates gradient data to the Rizer, not the application.
Configuration of the Rizer to Zwift is needed only for the steering function.
To configure the accessory, follow this procedure:
Run Zwift.
Click on the STEERING icon to open the search window.
Select Sterzo or Rizer in the search window
When the Sterzo icon turns blue as shown in the picture it means that the Rizer / Sterzo has been successfully configured and you can start your training.
If you notice during training that the avatar is not following the steering angle correctly, you can calibrate and centre the angle. To do this, simply, from the Zwift main screen, go to connected devices and press on the spanner next to the Steering/Rizer box.
More details can be found at this link:
https://support.zwift.com/en_us/steering-with-a-sterzo-smart-SyAN1Debv