1. Select your project type, CARVER SATURN ONE will show up if it’s connected to your computer correctly.
2. Go to " Garageband" >" Setting", choose " CARVER SATURN ONE" as input and output device.
3. Enable input monitor to hear your input signal in Garageband, you can also hear the direct input signal by activating the Direct Monitor button on the top panel of Saturn Two.
Further Reading:
Getting Started with Garageband (Tutorial Video)
https://youtu.be/d8aSDSGz8t4