I know this is an old bump....but it might help somebody else.
Mexican food in Atlantic City begins & ends with Mexico Restaurant:
3810 Ventnor Avenue, Atlantic City, NJ 08401-5945 (609) 344-0366
It's authentic, very reasonably priced...and I've never had a bad meal there. Keep in mind it's very casual. The decor is a little old, but it's a clean place.