You have to provide more info on what went wrong, because by default the Ubuntu installer already installs grub to the MBR of your hard drive, no need to do that afterwards or to worry about the right version of grub.
As to rEFIt, this can be installed in several ways. Easiest is to boot Mac OS X, download the latest .dmg from the rEFIt site at SourceForge, then install it with its own installer using all the defaults. Open a terminal and enable it with the command
Rebooting shows the rEFIt boot menu with an Apple icon (the default) and a Penguin icon (for Ubuntu). Don't boot the Penguin just yet: use the arrow key to select the rEFIt Partitioner Tool to synchronize the MBR and the GPT partition tables; then choose rebooting the computer. The next time the rEFIt boot menu appears, the Ubuntu icon can be chosen to boot.
$ sudo /efi/refit/enable-always.sh
Some people triple-boot with Windows as a third OS, i.e. they install Windows after Ubuntu, thereby overwriting the MBR. This requires reinstalling grub to the MBR and redoing the synchronizing bit. Let us know if this is your problem.