Forbidden sign when I press the backlit button on my keyboard.

My understanding from playing around with it is that that symbol shows up when the backlight isn't currently on, due to the ambient light being too bright.

You can test this by covering both speaker grilles with something (heavy paper, for example) that blocks the light. (The light sensors are under the speaker grilles.) Your screen will probably dim, depending on your setting in System Preferences -> Displays for adjusting brightness as ambient light changes. Then click those keys again. That symbol will be gone and the keys will do what you expect.

The sign will only appear when the ALS detects that it is too bright for the backlit keyboard. You can only adjust the brightness of the keyboard in low light conditions but as has been said, cover the speaker grills and you can adjust it then, but note the light will go off if you remove the cover over the grills.

hope that helps

Callaghan001

Actually there´s a way to force the backlight on, off or any level you may want.
check it out it´s really useful.

http://www.apple.com/downloads/macosx/systemdiskutilities/labtick.html

hugs from Spain

When the light is off you will get that. You cannot control when to turn off and turn on the light. The system does that through the light sensors in the speaker grilles as mentioned. When the system deems it needs it, it turns it on however you are only then able to either turn it off yourself, or modify the brightness. You cannot force it to turn on.

Thanks! Didn't know about that one.