I did use the T.C. Helicon Harmony M with the S970. However, I'm afraid I don't recall how I set it up. It wasn't difficult though. They both must be on the same midi channel of course. You will find that in the TRANSMIT section on the midi menu on the Yamaha. Set it to transmit on channel one, then be sure other channels are not transmitting date and save it as a User setup.
It is also best to disable Program Change on the T.C., or the keyboard can send it some strange messages. I kept wondering why my Reverb was being reset, and that was the culprit.