If you have the Windows Backlit Keyboard
On the top right hand side of the keyboard you should have 4 lights. Press the 'On' button to turn on the backlighting and 'off' to turn off the backlighting.
The two extra keys to the right of these control the level of the lights. (there are three light modes.)
If you have the Mac Backlit Keyboard
On the top right hand of your keyboard above the number pad are two backlight keys. Simply press these to control the light levels.
PLEASE NOTE SLIMLINE KEYBOARDS ARE NOT BACKLIT. (Only the 'backlit' models have back lights)