Did you get it to work? I have two Mac computers (Macbook Air and Macbook Pro) and neither of them seem to be able to identify my Forerunner 645M. When I connect the watch, it asks whether I want to use MTP or Garmin, but in both cases, nothing except charging happens. Garmin Express doesn't see the watch, and it doesn't show up in my Finder. I have tried SyncMate, but that app doesn't see the watch either.
I did get it to work, using Android File Transfer. That's the best way I found on a Mac for exploring and manipulating all the watch files and directories, but it conflicts with the Garmin USB driver. So whichever one you use, you must ensure that the other one is not running in the background.