If a MIDI device doesn’t appear when it’s connected
If you open Audio MIDI Setup then connect a MIDI device, Audio MIDI Setup should detect the device automatically. If the device doesn’t appear in the configuration pane, you may need to rescan the MIDI connection for the device.
Choose Window > Show MIDI Studio.
In the MIDI Studio window, click Rescan MIDI.
If the device still doesn’t appear, make sure it’s properly connected to your Mac, that it’s turned on, and that you installed any software required by the manufacturer.
If you delete a MIDI device and then decide you want to continue using it, you need to rescan the MIDI connection again to see the device in Audio MIDI Setup.