NBI doesn't support brightness control keys on a Dell mini 9. That means OSX does not intercept FN+8/9. Instead brightness control with these keys is done completely by the BIOS.
(Proof: When you use the volume control, the typical OS X volume bezel will show, when you use brightness control, there is no bezel.)
In your case, try the following:
- boot into another OS (i.e. some live linux system from USB) - can you use the brightness control there? if yes, its the OS
- if not, try to upgrade your bios, or try to "repair" it with the same version