The regular owners manual has instructions on Adjusting the Drive Belt. He's a brief re-cap if you have lost yours.
1. Open the rear door and disconnect the negative battery cable
2. Remove the 3 belt shield fasteners and remove the shield
This is hard to describe without pictures: There is an idler pulley that provides tension. Between it and the block is a little indicator pointer which shows amount of tension. When the pointer is in the 1 o'clock position, there is no spring tension on the pulley. When the pointer is in the 3 o'clock position, it hits a stop at maximum tension.
3. Loosen the bolt to adjust the spring loaded drive idler (You'll see a bracket with a slot to allow travel for adjustment, this is it)
4. Push the idler pulley against the belt using a pry bar. The indicator should travel to the 3 o'clock position at full tension, then let it raise a little bit so that it is not against the stop
5. Tighten the mounting bolt to 25-28 ft.lbs
6. Run the engine for a few minutes and check the tension again.
7. Re-install belt shield and fasteners.