My Husqvarna mower just had a new drive belt installed but now the belt won't move when engaged and starts to smoke. Any help would be great

By engage do you mean, will not move the tractor? If this is so, the belt is not installed correctly. Check to make sure all "v" shaped pulleys are running inside the belt and flat shaped pulleys on the outside of the belt.

If drive belt is to short or to tighten

sounds like the seat safety switch is not working. Check to see if both wires are connected to that switch. If they are.. then disconnect them, jump them together and see if mower runs Ok

Sounds like a pulley key is missing. That will cause the pulley to spin on the shaft without the shaft turning.

If it's a JD rideon make sure you have pushed the lever back in all the way!

could be engine timing not right

first check belt then check the idle pully

If the main drive belt, from the engine to the rear end of the tractor broke, you won't move, nor will you hear a whining or grinding noise!
An easy check- inspect if each rear wheel has a key installed between it and the axle shaft. One pops out- it won't drive.The key is a square piece of hardened steel that is inserted into the axle, then the wheel hub is slipped over it and the axle, then the necessary fastener is installed to keep the wheel on. This could be either a bolt, or a spring clip. And, if the transmission has failed, you may or may not hear any whining or grinding noises!

My guess it the drive belt is toast as you said it was smoking, look at it and the order a new one. Here is a site you may be able to download a manual, Tim:

http://www.searspartsdirect.com

the belt is smoking because a pulley isnt spinning like its supposed to. its gonna break the new belt if you don't remedy the problem. try taking the belt Back off. then see if you can spin the pulleys by hand with no belt .