Belt won't stay on belt sander

Check first that you have retensioned the belt... i.e. when you removed the other belt, you would have had to pull on a spring-loaded lever to remove the belt tension before you added the new one... make sure the tension lever is flipped back in to ensure the new belt is tensioned...

Next, and the likely problem you are having, is belt tracking... There should be an adjustment knob up near the front of the sander near the front roller, either on top of the front part of the sander, or to the side near the front roller... When you turn this knob, it makes the belt track either left or right over the sander's base. This knob will allow your belt to run square and true and not spin off to one side...You need to adjust the tracking using this knob while the belt sander is running and with the new belt tensioned and in place, so turn the sander upside down, and power it on with one hand, then turn the knob with the other hand whichever way is needed to have the belt track square and in the middle of the rollers. Make small adjustments on the tracking knob to begin with. They can be sensitive at times.

That should stop your belt spinning off to one side.

This belt sander is a POS and should not be purchased by anyone. Just do a Google search for Black& Decker Dragster belt slipping. EVERYONE has this problem with this sander and the fix is a mod with a Dremel tool. Mine also has the problem of the belt slipping because the tensioner is not as effective as it should be.