I don't have any issues mapping B-5 V2 in MainStage 3.3.
What issues are you experiencing?
I'm a PC guy with no Mac/Mainstage experience, but is there a reason not to use the MIDI mapping within B-5 / UVI Workstation? It works great for me.
No reason whatsoever if you are running UVI Workstation as a stand alone or in a host in Windows.
The work flow in MainStage is a little different in that you map your controller in the Workplace in MainStage, which gives an onscreen visualization of your controller, and then map these screen controls to each VI, like B-5,in MainStage.
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