In Abletons midi settings you have to activate sync on the Rev2s output port (that could be named Rev2 if you connect it through USB, or the port number of your midi interface if you use one).
Further more you have to hit the play button in Ableton so it will start to transmit the midi clock. (Perhaps there's an option to configure Ableton to transmit the clock when paused, but if so, I didn't find it yet.)