What is the MIDI software you are using on your Mac?
If the MIDI guitar only has a USB MIDI port, what you’ll have to do is make your Mac forward the USB MIDI data from your guitar to one of the 4 Midihub’s virtual USB ports, process it, and send it back via USB.
Most DAWs have functionality to forward USB MIDI data.
There’s also MIDI Patchbay for Mac OS that can help with the forwarding. The latest version is here, not sure how to go about installing it though. 