[Yosemite Sam]

My first impression is that you have a deck spindle that is stuck up or possibly the idler pulley on the deck.

Next guess would be that one of your mule drive pulley bearings is bad or the mule drive is bent.

It is also posible that when installing the belt it came off of one of the mule drive pulleys and got cought between the pulley and it's bracket (I have done that myself)

With the pto belt off check your deck, turning it by hand make sure all three spindles and the idler pulley are turning easily, check the center pulley (top one) to make sure it isn't bent or loose. Reach under the deck and grab the center blade, try to make the center shaft rock in its bearings by moving one end of the blade up and down.

Then check the pulleys on your mule drive, make sure they spin easily but do not wobble and are not bent or crooked.

Then check the pto, make sure you can turn the pulley (with the engine not running) by hand while the handle is pulled back. Then move the lever to the forward position and make sure that the pto assembly is not loose or wobbely and that the pulley will not turn unless the the engine turns with it.

You probably want to check your belt too, to make sure that it isn't chewed up or have any bad spots in it.

When you put the belt back on, make sure the center pulley isn't pointing downward in the front, a mis adjusted deck will also cause the belt to come off.

If all these things are good then it's hard to tell what the problem is. I have seen situations where everything including the belt appears to be good but the belt just wont stay on. In many cases like that a new Cub Cadet belt of the correct length will solve the problem.

The white smoke makes me think that either the belt wasn't on one of the mule drive pulleys or something is stuck or bent.