If you attempt to use the MIDI IAC driver on OS X and receive the error:
MidiOutCore::openPort: error creating OS-X MIDI output port
then most likely some resource has already reserved the MIDI port in question. The MIDI protocol in general does not handle concurrent connections well. If you have anything using the port (for example, the MIDI Monitor http://www.snoize.com/MIDIMonitor/ ), then the MIDI port may already be reserved and NI mate will be unable to attach to it.
You can solve this problem by closing the software that’s currently occupying the MIDI port.