I've got a 2004 malibu classic with the 2.2. I can't seem to find the belt tensioner. please help.

It's just to the left of the alternator pulley. Place a 3/8" ratchet or breaker bar into the square 'hole' on the arm and crank it CC to release tension.
http://media.gm.com/us/powertrain/en/product_services/2008/Photo%20Library/Ecotec/L61/08%202.2L%20I4%20L61%20MAL%20LoR.jpg

Joel