The brightness control keys on a ThinkPad are usually Fn to increase the brightness and Fn to decrease the brightness. Some other thinkpads also contain a slider on the side of the monitor that will allow you to do this.
On alot of laptop's there is a function key to do this. It's usually blue and down by the ctrl/alt area. Press this and hold the corresponding F key. there should be one to increase brightness and decrease it.
