Omamez-
Fstab should already be setup to automount. So, you don't really need to edit fstab if you plugged a drive into the usb port.
Edit this file here > /etc/emulationstation/es_systems.cfg and change the rom paths for the emulators you want to use the external hd. It should probably be something like this /media/usb0/roms/mame. Basically, /media/usb0/ will be the root dir.