The Ubuntu documentation page has the following information in relation to the Macbook.
LCD Brightness Control
The package nvidia-bl-dkms is not available for 12.04. Fortunately, the following procedure works (taken from http://www.voria.org/forum/viewtopic.php?f=3&t=1091):
If backlight control does not work for you, follow these instructions:
Edit the file '/etc/X11/xorg.conf' and add this line into the Device section:
Option "RegistryDwords" "EnableBrightnessControl=1"
Reboot and check if backlight control works. If so, you're done.
If not, edit the file '/etc/default/grub' and change the line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Update grub with this command: sudo update-grub
Reboot and backlight control should work.
If it still doesn't work, try installing the nvidia-bl-dkms package from mactel-support for Natty (from https://launchpad.net/~mactel-support/+archive/ppa), this was the only way for me to get the backlight control working on Precise. Results may vary as some people claim this driver does not work on Precise.
Link to these info is below.
Also this blog appears to have used a script to get it to work.
Best of luck!