If you are working with Pure Data, you don’t need Ableton to be involved. PureData talks directly to the system Audio and MIDI devices, so whatever MIDI interface you have should do the job.
If you want the OT to send MIDI to Pure Data, run a MIDI cable from OT’s MIDI OUT to the interfaces MIDI IN. If you want the OT to be able to listen to Pure Data, run a MIDI cable from OT’s MIDI IN to the interface’s MIDI OUT.