Connect the MOTU to the Macbook. Go to Apple menu > System Preferences > Sound > Input. Does the MOTU show up there as an available input? If so, you should be able to select it in Audacity’s Device Toolbar.
Similarly, in the Input section of Sound Preferences, does the MOTU show up as an available output? If so, you should be able to select it in Audacity’s Device Toolbar as an output.
Due to the updated security architecture introduced since macOS 10.13 High Sierra, the system extensions for all newly-installed third-party software will automatically be blocked from running. If your MOTU interface is not showing up in Audio MIDI Setup, MOTU Audio Setup, MOTU Discovery, or your DAW on High Sierra or later, you might need to enable the driver in your System Preferences.
Download and install the very latest installer for your MOTU interface.