The problem lies in the fact that once the belt has broken, the tensioner falls to rest flat against the floor of the dryer. When in place with the belt installed, the tensioner is actually almost vertical. Also, when installing the belt, the amateur repairman (like me!) thinks that the ribbed side of the belt must rub against both the tensioner and the pulley, and tries for an hour or so (like I did!) to figure out how to get the belt physically around the tensioner and the pulley and the drum, ribbed side down. This is not so. The non-ribbed side of the belt rubs against the tensioner, and the ribbed side wraps around the motor and the drum.
Here are three pics for you.
http://img.photobucket.com/albums/v621/klep26/IMG_4549.jpg
http://img.photobucket.com/albums/v621/klep26/IMG_4548.jpg
http://img.photobucket.com/albums/v621/klep26/IMG_4547.jpg
Hope this helps!
Rob Klepper, December 2007