So to get the screen brightness keys working with your Nvidia graphics card, create a file in the xorg.conf.d directory, e.g:
sudo gedit /usr/share/X11/xorg.conf.d/10-nvidia-brightness.conf
Paste the following into the file:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "Quadro K1000M"
Option "RegistryDwords" "EnableBrightnessControl=1"
EndSection
Log out and log back in, or reboot, and your brightness keys should now work!