I got a 4 year old craftsman (Model # 2885246) yard tractor and it does the same thing. To put the belt on, remove the battery and reach down into the rear pulley and put it back on. You will need to make a guide to keep the belt from popping off the rear pulley. I use a 10 inch bolt that is bolted to the right hand side of the frame. It sticks over the belt just enough so if the belt wants to bounce it won't jump off. The sears tech told me the machine was redesigned and the rear pulley was changed and they omitted the guide that was on the older pulley system. This fall it popped off in reverse, so I added another bolt which in below the belt.
Sears machines went down hill. I won't buy another if they continue to make them like this.