Idea... You first really need to verify that that directory path exists for you. You need to adapt that command to the directory structure of "your" system. That is the basic structure if your system has an Intel GPU. Go into Nautilus or command line and trace the tree down /sys/class/backlight/ and see what directories ar ender that that would indicate a GPU type name... Different systems with different GPU's have different names under that directory path.
Your going to see something similar to this... with whatever that "_GPU_directory_name_" below backlight, varied to what your gpu is...
As you can see, after that directory name, you will then see a four registry files named: actual_brightness, bl_power, brightness and max_brightness... each containing numeric values
First check the value in your registry via (remember to substitute for the directory name on yours):
The try changing the value and then check that value again to verify that it did change that registry entry...
Play with the values, noting that some laptops (Acer is one) have their values flipped. I have one where the values and controls work in reverse. <Fn><Up> is supposed to get brighter, and on mine <Fn><Down> is brighter...
If it changes the values in the brightness registry file, but does not affect the brightness of the hardware, then report it to launchpad.
Is that enough info?
#Note to Apple users- That won't work for you because of a bug with the ACPI Interface. Change yours in MACX and reboot into Ubuntu. It will read the persistent change from the system hardware.