For anyone on OS X who has M+GUI crashing after updating MAME:
Open a Terminal prompt, navigate to your MAME folder and type ./mame64 -cc (./mame -cc if you're on 32-bit OS X obviously) to update your mame.ini. Your existing settings will stay the way they are.
When doing this, you will see one of the weirdest error messages in the world:
"Illegal integer value for numprocessors: "auto"; reverting to auto"
Incidentally, this error message is apparently part of the reason why M+GUI crashes while populating the gamelist:http://www.mameworld.info/ubbthreads/sho...part=1&vc=1
In the Finder, click Go, Go to folder, type ~/.mamepgui and press return. Delete the cache folder and mamepgui.ini.
Move your mame.ini out of your mame folder.
Start M+GUI and point it to your mame binary as prompted. Once it's done populating the game list, quit.
Put back mame.ini and start M+GUI.
This way, if you had everything set in mame.ini already, you only have to set up the GUI part of M+GUI again.