Please make sure that you have the Quickset utility installed on the system. If not you can download it from the link http://dell.to/11XpyPA and follow the onscreen instructions to install it.
When the Quickset utility is installed, it will provide a graphical view of volume, brightness, and wireless adjustment. You can adjusts the LCD brightness using the key combination of 'Fn' key and 'Up/Down Arrow' key, the Quickset utility will provide a graphic popup of the adjustment, as shown below:
Hope this helps. Please reply if you have any questions.