The tensioner is right next to the alternator, between the alternator and the blower pulley. It is a bolt that pushes the tensioner pulley down and has a lock nut against the tensioner bracket. Importantly, the moving block of the tensioner (which the bolt pushes down on) has two bolts that tighten to the bracket. You must loosen these two bolts before adjusting the tension, and tighten them after that. They lock the tensioner in place.
So loosen the two large locking bolts, apply tension with the tensioner bolt, then tighten the two locking bolts. The 1/2" gap that the instructions refer to are about the moving block of the tensioner itself. It is split in two pieces with two spring-loaded pistons between them. As you apply tension, the two pieces start to compress; when that gap is about 1/2", you have applied the right tension and can then lock the tensioner block in place. Be sure to check it later if you put in a new belt because they seem to stretch at first.