Currently Being ModeratedJan 5, 2013 1:13 PM (in response to stephen kemp)
I noticed this in MainStage 2.2.2 as well. The button/footswitch/whatever needs to be assigned to a channel for the 'Momentary Behaviour' checkbox to appear. It looks like a bug to me. MS should not assume that I will use a MIDI controller. I might just want to click on the button onscreen.
I just use IAC 2 as NULL and send it there. You could also pick an unused channel, but be careful that it is actually unused.
Hope that helps!