First modify /etc/defaults/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux acpi_backlight=vendor"
sudo update-grub
Second, in the file /etc/X11/xorg.conf, find the in Device section and add Option EnableBrightnessControl, so it looks like this:

Section "Device"
     Identifier     "Device0"
     Driver         "nvidia"
     VendorName     "NVIDIA Corporation"
     BoardName      "GeForce GT 330M"
     Option         "RegistryDwords" "EnableBrightnessControl=1"
EndSection