Please note that the Audio Unit (AU) plug-in format does not support MIDI Learn on effect plug-ins, so you will have to use Smart Controls assignments:
- Load your UVI plug-in as an AU effect in a Logic track (for e.g. Rotary)
- Select Show Smart Controls from the View menu
- Click on the Information icon in upper-left corner
- Click the parameter you want to assign in the Smart Controls window (for e.g. Rotary Speed Mode)
- Go to External Assignment, click Learn then move any encoder on your MIDI device
Your parameter is now assigned to this encoder on your MIDI controller.