Start at the posted coords or just go to the corner of Geary and Masonic.
Letterbox Clues:
Usually "X" marks the spot for a hidden treasure. In this case, we start with a bullseye. Whatever your path, find your way to C Level of the parking structure, home to The Nerd Brigade... or something like that. Between Level C and D is a ramp with a sign. To the left of that sign is a stairwell. Resist the temptation to go down and relax in the cool seating area below and look for a strange metal structure on the floor just next to the top of the steps. Above that is a nook in the I-beams that you'll need to duck to get access to. On Hollywood Squares, Paul Lynde usually occupied the center square. You'll find him here as well.
Circle gets the square: http://www.youtube.com/watch?v=fSkRsciX-gI