OK, well, I mounted my magnet to the brake disc...the sensor goes on the inside of the caliper bracket (forward of the caliper itself) with a bit of vacuum hose as a spacer to get it close to the disc. Had to drill a tiny hole in the disc itself, as well as one through the caliper mount(on the fork leg).
good luck, hope it gives you some idea's.
Andrew
One more thing ...... If your magnet is not a replacement bolt for the rotor (why elese would you ask), it just gets epoxyed into a slot on the rotor. Trail Tech sells the rotor bolt replacement magnets. I have no affiliation with them, it just worked well on my bike. http://www.trailtech.net/accessories.htm