If you have just created a dual-boot system with Mac Snow Leopard and Ubuntu Karmic and find that the Grub2 does not boot up your Mac, you can follow the instruction below to add the correct boot entry in Grub2.
Boot into Ubuntu Karmic.
Open a terminal and type:
Add the following to the end of the file:
Save and close the file. Back in the terminal, type:
That’s it. You should now see an extra entry in your Grub screen. Scroll down to the last entry and it will boot into your Mac Snow Leopard.