I'm trying to put a deck belt on a 42" MTD, 14.5 hp. All the numbers on the labels are worn off so I cannot get an owners manual or schematic. We have dropped the deck down and have gotten the belt on 2 pulleys, (there are 3) but it appears short and will not go on the other pulley. I took the old belt in to Tractor Supply and they measured and gave me the correct size. Is there some tension adjustment for this or something? Thanks for ANY help you can give me.

Make sure the blade engagement handle is in the "disengaged" position. If you are working on the two deck belts themselves, you may need to remove a spring, get the slack needed and put the belt on.
Note***
Trick to remove and install springs, first use eye protection.
Then use a piece of small pull starter rope cord and run a loop
Around the hook on the spring, you can pull the spring open further
This way for putting on, or removing the spring.

Try this site and go to customer service for a manual. MTD Corporate :: About MTD Products I would only use OEM belts but I am fairly sure you have at least 4 pulleys for your deck. Normally two more pulleys than the number of blades. The extra pulleys are an idler and a spring loaded tensioner that does have some adjustment. You should not have to remove the springs if you get the right belt from the start. Belt and configuration are usually a decal on the deck but that could have worn away by now.