Before you go into the large room with the puzzles, there should be a circle before the door. Start from the centre of that circle, and go in the direction away from the door. 6 steps down, and then 9 steps right. When you activate the switch on that tile, the circle before the door will glow and when you go through, go past the clues to the back of the chamber and Regirock should be there for you to battle.
Link - a video walkthrough
Serebii - list of legendary Pokemon & how to obtain them