Mapping a MainStage control

I use the Pianoteq AU plugin with MainStage for live performance. I would like to map a control to the arrow that moves me from one Piano to the next in the list of five favourites, in the same way that you can normally map a control to some parameter of a plug-in. I can't get it to work. MainStage is clearly mapping something to something as, when I click the arrow, the on-screen button light flashes. But clicking the on-screen button doesn't change the piano to the next in the list. Anyone else tried this and got it to work?