I would definately clean the deck off and make sure idler pulley is free.
Adjusting belt tension- PTO lever has to be in OFF position. Behind left side(if you were sitting in seat) blade pulley cover there is a small bracket that holds the cable and the cable connects to a spring. There are two nuts(looks like small bolts to me) that holds the bracket to the deck. Loosen the nuts and move bracket away from center of deck to tighten belt tension. Move bracket until almost all slack is out of cable but cable is not too tight, then retighten nuts.