MOTU does not currently have drivers for our serial devices to be used under OS X.
Using a third-party serial port or serial-to-USB adapter would be reliant on the manufacturer of that serial port/adapter providing a driver for MIDI I/O. That would also mean that even if that manufacturer provided for some sort of MIDI functionality, you would not be able to use the sync capabilities of the MIDI Timepiece AV, Express XT, or Micro Express, or the networking ability of the MTP AV. Regardless, we cannot guarantee proper MIDI functionality with a third-party’s driver, nor provide support on attempts to get such a system to function.
You can still take advantage of your MIDI Timpiece I, II, or serial MIDI Timepiece AV by networking it to a USB MIDI Timepiece AV. Check the link below for help on setting it up. To use your Digital Timepiece under OS X, you can make bi-directional midi connections to a USB MIDI interface.