On the second level, you'll start seeing red boxes (with a white t-shaped lever in them) on the walls; these are the fire alarms you want to pull. Pull 1 for this achievement, 10 for a second achievement, and all of them for a third achievement.
P.S. Not to be confused with the red boxes high on the walls you see from the first level (with a white horizontal bar on top) -- the fire alarms are generally shoulder-height from the ground with the t-shaped lever and grey circular bit above.