Its true, when the computer is in a bright room the lights will not turn on. This is probably because it is useless in daylight. It can almost not even be seen and if one was to use it on battery it would drain it quite fast.
You can hold your hand over the ambient light sensor and activate the light by pushing the light up button (F6). The ambient light sensor is located a few millimeters to the left from the iSight camera on your MacBook Pro unibody.
If you have a MacBook Pro that is not unibody there is two ambient light sensors, each one under the two speaker grills on the sides of the keyboard.