OMS requires the latest drivers for the MTPAV to be installed.  In order do this properly, you must install OMS _first_, restart, then install FreeMIDI.  *(if OMS is installed first, freemidi's installer detects OMS and places drivers for the MTPAV into OMS system components).
 If Freemidi has been installed _before_ OMS, use Sherlock to find "Freemidi" and trash any item that says freemidi.  Then restart the computer, and perform and "easy install" of FreeMIDI.

Here are the exact installation steps for installing properly:
1. Trash all FreeMIDI, OMS and Opcode items, also search for Open Music Systems.
2. Install OMS - and make sure it's version 2.3.8
3. Allow the computer to RESTART after OMS installation is complete.
4. Install FreeMIDI - and make sure it's version 1.46
5. Allow the computer to RESTART after OMS installation is complete.
6. With the MTPAV on and connected to the computer, launch OMS and let it scan and hopefully find the MTPAV.
If you follow these steps carefully, OMS should see the MOTU midi interface.

More things to check:
A. make sure the MOTU midi interface is ON
B. reset the MOTU midi interface to factory settings by holding the PANIC button while powering the MOTU midi interface on. *(fastlane has no reset).  Hold it for a few seconds, until the screen says "Base Setup"
C. Verify connections.  If you're using Mac serial ports, try a different port.
D. Use the latest version of FreeMIDI, available as a free download at motu.com .  Freemidi is updated regularly, so check often.
E. 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 MOTU midi interface directly to the Mac (USB).  See if FreeMIDI detects the MOTU midi interface without the hub.
F. disconnect peripheral USB devices (printers, scanners, etc), and disable their drivers in extensions manager.  After restarting the Mac, try FreeMIDI again.