Problem
When you press F5 or F6 to adjust the backlight settings for your keyboard it displays the expected image but with a no entry sign underneath.
Solution
The backlit keyboard enables itself based on the current light conditions. You can test if it’s working by putting your thumb over the iSight in-built webcam and then trying to adjust the setting.
You can also download a program called Labtick which allows you to adjust the keyboard settings whenever you want.
None of the above works. Backlight does not function in a dark closet on my MBP. Installed Lab Tick still nothing.
Then it sounds like you have a problem with the hardware. Take it to your local Apple Store or Authorised Centre.