Looks like you might need to try nvidiabl then. Open a terminal window and execute these commands:
...post back the results of those commands.
sudo dpkg -i nvidiabl-dkms_0.80_all.deb
sudo modprobe nvidiabl
Hopefully, everything will install normally, but the function keys may not work right off the bat. To test whether you can change the brightness:
1. Get your current brightness value:
2. Get your maximum allowed brightness value:
3. Set a new brightness value:
...where XX is a value between 0 and the result of step #2.
echo XX | sudo tee /sys/class/backlight/nvidia_backlight/brightness
If you can change your brightness manually this way, try your fn function keys. If they don't work, we can try mapping them.