I have a floor standing Porter Cable drill press that looks like it has a similar pulley system. The fact that the tensioner wont move at all makes me think they are locked. On my model there are two thumbscrews on the side of the drill near the top at the back. Make sure those are loose and then try your tensioner again.
Failing that I would look into getting a better sized belt, there are belts sold that are linked segments instead of one solid piece so you should be able to tweak the length to a degree. Also I dont believe the whole motor moves, there is an "L" shaped bracket that the rear pulley is mounted on that will rotate to tension the belt, shown in the idagram as the lower part of "OMVE".
Failing that I would look into getting a better sized belt, there are belts sold that are linked segments instead of one solid piece so you should be able to tweak the length to a degree. Also I dont believe the whole motor moves, there is an "L" shaped bracket that the rear pulley is mounted on that will rotate to tension the belt, shown in the idagram as the lower part of "OMVE".