Is it an FX15, or FX720/730?
With the FX720/730, you screw the mounting bolts all the way in, back one off slightly, use a wrench to turn the spring tensioning nut (the ring with the flats on it around the head of the mounting bolt) so that it pulls the pad away from the rim, then tighten that mounting bolt while holding the nut in that position. Repeat on the other side. You fine-tune the balance by putting a wrench on a nut, loosening the bolt and moving the wrench slightly to give more or less spring tension so the pads are equidistant from the rim, then re-tightening the bolt.
I think the FX15 is just tightened down with the pads as far from the rim as possible, then you adjust the balance by screwing or unscrewing the grub screw that controls one side's spring tension.