Press F6 key.
System Preferences > Keyboard
Uncheck and recheck the box next to "Adjust keyboard brightness in low light".
Reset SMC. http://support.apple.com/kb/HT3964
Choose the method for:
"Resetting SMC on portables with a battery you should not remove on your own".
"Mac notebook computers with non-removable batteries like the MacBook Air do not require regular calibration."
If your portable Mac has a built-in battery, you don’t need to calibrate the battery.