Here are a few things to check
- make sure the MTPAV is ON
- reset the MTPAV to factory settings by holding the PANIC button while powering the MPTAV Oon.  Hold it for a few seconds, until the screen says "Base Setup"
- Verify connections.  If you're using Mac serial ports, try a different port.
- Use the latest version of FreeMIDI, available as a free download at motu.com .  Freemidi is updated regularly, so check often.
- If you're using a USB hub, be sure that it's self-powered, and that the drivers are the latest.  Try bypassing the hub and connecting the MTPAV directly to the Mac (USB).  See if FreeMIDI detects the MTPAV without the hub.
- disconnected peripheral USB devices (printers, scanners, etc), and disable their drivers in extensions manager.  After restarting the Mac, try FreeMIDI again.