There is a lever on the brake mechanism that has two holes in it, to which the brake rod should be connected. Make sure the rod is connected to the hole with the most leverage (the one closest to the end of the lever).
There is a bolt/retaining nut the comes out of the side of the brake. Loosen the retaining nut and thread the bolt in until the pads are now touching the brake disc. Back out the bolt just a little so there is just a little play in the caliper and the wheels can turn freely, but not so much that the braking power is crap. After you find the right position for this bolt, tighten the retaining nut and you are ready to go.
Properly adjusted, your brakes will be able to make the tires skid on pavement when you stomp on the brakes.