I need a belt diagram for a 42 inch MTD riding lawnmower.

Isn't that like asking for the plumbing diagram of a house?

Ray, MTD and others usually put a decal on the deck showing the route. You have 2 more pulleys than you have blades. The two extra pulleys are an idler and a spring loaded tensioner. You must disengage the blade by pulling on the handle and then pull back on the tensioer as far as you can. This is frequently a two man job. There are "keeper pins" that help the belt stay on the pulley. You can force the belt past them and even then you will need to peel the belt onto the last pulley.

One thought here though, be careful when messing with the tensioner pulley, it can hurt if it slips.