I had the same problem. You’ll need to adjust your rom directory in the mame.cfg file to point towards the RetroPie rom path.
rompath=/home/pi/RetroPie/roms/mame-mame4all
Provided you’re using the default location I suppose. Also, you’ll need to point to the RetroPie config path too.
cfg=/opt/retropie/configs/mame-mame4all
I hope that helps.
-Ryan
Fantastico! That’s me up and running now! Well ALMOST! I last thing is that it’s not saving my inputs so if I quit the emulator I have to go back in to the TAB menu to configure the controls.
Guessing this could be a permissions issue perhaps?