Just installed the tool myself today. Ran into the same problem, same midi device. I found a fix:
1. Open location of your Lightroom modules: (~/Library/Application Support/Adobe/Lightroom/Modules)
2. right click midi2lr.lrplugin, 'show package content'
4. right click on midi2lr_BAK.app, 'show package content'
5. now you see a finder window with ANOTHER '
midi2lr.app'
6. move this
midi2lr.app next to the midi2lr_BAK.app by opening dedicated windows for the location
This fixed the issue for me. Open Lightroom to check if the app is opened automatically.
Seems to be an issue with the installer for mac.
Let me know if this works for you, in this case i will open an issue on GitHub