Try this:
- Open up display preferences.
- Uncheck the "automatically adjust brightness" box
- Set the screen to the brightness level you would like to see it at by using the slider
- Now Check the "automatically adjust brightness" box
What this does is set your viewing preference, which should then be adjusted based on the ambient light to maintain the same relative level.
You can quickly test if it is automatically adjusting by either covering the facetime camera up with your hand and looking for the screen to dim and the brightness adjustment slider to move to the left OR you can shine a flashlight at the camera and the screen should brighten up and the slider should move to the right.
If the screen brightness doesn't change when doing these tests, chances are the slider isn't moving either, and there is a problem with the ambient light sensor itself. If this is the case, I would recommend doing an SMC reset. See the following Apple support article: http://support.apple.com/kb/ht3964
If the SMC reset doesn't fix the problem, then you have essentially exhausted your at home troubleshooting options, at which point you would need to take the computer to an Apple store and see if they can help.
Hope this works for you!