Thanks Arno, I got it to work, it is little more convoluted process for button controls than other VI's. The 'midi learn' feature in B-5 does not work for these controls, for example CC 101 or button 6 on my M Audio Axiom 2nd gen attempting to map to 'percussion on off'.
The way to get it to work in MainStage is to activate 'assign and map' in MainStage, then depress the button to be mapped on the controller - CC 101 in this case, and then click on the percussion 'on off' in B-5. Then deactivate 'Assign and Map' in MainStage.
I just checked and the midi learn feature in B-5 does work for rotary encoders and faders, for example rotary encoder CC75 to tube distortion.
I would recommend you test B-5 with MainStage, it is only $29 and is the host of choice for live performance on Mac's.
Using MS 3.3, B-5 v2.0.2
MainStage 3 | Axiom 61 2nd Gen | Pianoteq | B5 | XK3c | EV ZLX 12P Hoping everyone stays safe and well and sees this through to the other side