As others have mentioned, the allen nut is the recessed bolt on the other side. But usually for most dual pivot calipers like the ones the OP shows, the main allen bolt on the seat stay bridge is for bulk centering. We use this to orient the calipers near center and tighten. The fine centering can be done with a small allen or flat screwdriver on the recessed center set screw on one of the caliper arches. However, if the brake anchor bolt is correctly torqued and not over tightened, you should simply be able to rotate the arches slightly left or right and center them. Then squeeze the brake lever and suddenly, the brakes should be relatively centered. No tools needed.