To increase the brightness setting of your ThinkPad LCD display, press and hold the Fn key, then press the Home key. To decrease the brightness, press and hold the Fn key, then press the End key.
The Fn key is typically located at the bottom-left of your keyboard and looks like this:
Note: Some ThinkPad LCDs will automatically dim when the AC Power is unplugged. This is to save battery power and is normal.