If you are using advmame on a Pi2 with RetroPie 3, the config file is here:
/opt/retropie/configs/mame-advmame/advmame-1.2.rc
In that file, the rom path is set here:
dir_rom /home/pi/RetroPie/roms/mame-advmame
You can always change that path if you want.
