Realtime Messages ... How do they work?

I'm trying to use the MC6 to start and stop a synth and/or drum machine. I've programmed in the Realtime Messages, and when I press either I can see the "play" button light up on my synth, but it doesn't play. I've tried it with the synth on master or slave mode. When it's on master mode, the synth doesn't react at all, when it's on slave mode the "play" button lights up but it doesn't play. Am I supposed to have a feed of master tempo always running and if so, how do I implement this? Thanks!