Hi and thanks for your question, Patrick. One of the best ways to check what's up is to see if the Yamaha sends data back when you spin the tempo knob on it. Try downloading MIDI Monitor for iOS and see what happens when you spin the tempo knob. This could work. AFAIK there is no standard controller number for tempo.
On the other hand, perhaps the Yamaha is waiting for external MIDI Clock or MIDI Time Code. If that's the case, MIDI Designer doesn't do that (yet), but eventually will.
Thanks again for your question and for checking out MIDI Designer.
Best!
Dan