Loosen the two long bolts that tighten the clamp on the rear bearing carrier.
On the bearing carrier, you will see a hex form formed into the bearing carrier housing. You will need a wrench big enough to fit.
At this point, you will notice that the the bearing carrier is egg shaped, and as you rotate it inside of the clamps, the axle moves forward and back, thus tightening or loosening the chain.
There isnt much adjustment here, so if its not enough, it means your chain is too stretched out and needs to be replaced.