I was curious about this cheese, Ive often seen it in American recipe books.
A quick google search gave me this...
About Monterey jack cheese. An American cheese produced from cow's milk that may be skimmed or whole.
It has smooth texture and a very mild flavor. ...
Substitute for Monterey jack cheese - Muenster, Havarti OR Gouda.
It sounds like a colby cheese to me, smooth with not much flavour.
I agree with Jenna - most supermarket cheeses other than tasty should do the trick.
Vanessa