This setting gets buggered up when I make software changes. The red lighted button on the X-Arcade is "mouse 2". So you need to go into mame... or when a mame game is running: "Tab", Select "input (general)","user interface" ... scroll down to "UI Cancel" , "enter", [RED Button]. I have yet to figure out a simple way to add buttons (keeping the defaults) and adding "mouse 2"
You could also make or exit the file "MAME\cfg\defailt.cfg"
default.cfg