Help!! I can't figure out how to change the PTO belt (the one that engages the back tires) on my zero-turn 7500 craftsman mower.

Hi,
Those are new models. I have only seen them on the sales floor. Got a model number?
Peace,
Clarke

Model number 107.277740

Hi,
Well, that is a new vendor number!
The parts breakdown is at Parts & Accessories | Shop & Find Lawn & Garden, Appliance Parts at Sears PartsDirect | SearsPartsDirect.com (http://www.searspartsdirect.com/) and three manuals at Home Improvement Projects, Product Manuals & Home Management Tools - Managemyhome.com (http://www.managemyhome.com/) though I think I would probably settle for the one in the middle of the list.
It is probably a job for four hands in places. One set of hands to pull a length of rope attached to the idler arm and the other set of hands to put and keep the belt in place.
I say this because the spring on the idler is bond to be "stout." This may not be obvious to anyone not used to working with belts but as the picture shows; the sheave on the right hand transmission (left hand on the actual machine) wraps less half way around. A strong spring would be required to prevent slippage. The belt will fall off if not held in place, or would not stay on the various sheaves.
The deck belt must be removed first to gain access, and of course reinstalled after.
Peace,
Clarke

You must remove the PTO clutch, 1 bolt. 1 wire connector.
Remove mower deck first.