Command line mame is easy. If your roms are in /media/mmc2/roms you'd do-

~ $ xmame <game name> -rompath /media/mmc2/roms

hope this helps,
kernelpanic

<edit> forgot to mention- /edia/mmc1/roms is hardcoded. So if your roms are there,

~ $ xmame <game-name> will suffice...
</edit>