Ubuntu on Mac (dual-boot). Where do I install GRUB?

When installing Ubuntu, I choose to install GRUB on a partition other than:

/dev/sda i.e. /dev/disk0

as this is stated in various tutorials.

However, this way I end up with the error message: Missing Operating System

When installing GRUB in /dev/sda everythings works fine. Why?

@ mods: I hope this is not off-topic, as this a problem with the EFI boot on a Mac, I assume. If I'm wrong, please migrate this question to askubuntu.

Make a new partion from within OS X, then install http://refit.sourceforge.net/ and then ubuntu: https://help.ubuntu.com/community/MactelSupportTeam/AppleIntelInstallation