The designers of the Mamiya 645 Pro (and related cameras in that series) did not bother to include a threaded port for a cable release. You could purchase a ridiculously expensive adapter on eBay, or you could print this thing for less than 10 cents.

The adapter slips over the shutter button and permits the use of a cable release that you probably already own. For me, it printed best with small end down, .1mm layers, no supports.