Opcode does not directly support the DTP, but as usual MOTU has found a way to make it work. If you have installed a version of FreeMIDI other than vers. 1.43, remove it. To remove FreeMIDI, locate the following files and move them to the trash.
-FreeMIDI Applications folder (and it's contents)
-FreeMIDI folder (found in the System folder)
-FreeMIDI PowerPlug
-FreeMIDI SoftSynth
-FreeMIDI System Extension
-FreeMIDI Preferences

In order for the DTP to recognized by OMS, follow these steps IN THIS ORDER!

1. Install OMS 2.3.8
2. Install FreeMIDI 1.43 (or higher)
3. Run the OMS Setup program. When you scan for "MIDI Cards and
Interfaces" it will now recognize the DTP.

If you don't follow these steps in this order, it will not work.
Basically, FreeMIDI will try to install it's necessary parts into the System folder and the OMS folder. FreeMIDI installs a "MOTU OMS Driver" in the OMS folder (found in the System folder) that will give OMS the ability to recognize the DTP, MTP AV, and all newer MOTU interfaces.