I think that Monterrey Jack cheese is called for in Mexican recipes North of the Border because it is thought to be the closest to the types of white cheeses found here in Mexico. You may try looking in an Hispanic grocery store for "Queso Blanco", or if you llive in an area with many Latin people, it might be carried in your regular grocery stores. I was surprised when I got down here to find that authentic Mexican cooking does not use alot of cheese, except in quesadillas, of course, where they use a nice white cheese that melts easily. I think mozzarella or string cheese is actually closer than jack. Also, they crumble a nice fresh cheese called "queso fresco" on beans - it's like a mild feta. Very very good.