rEFIt has nothing to do with installing Ubuntu. it is merely a bootloader (like Chameleon)
Are you currently using the whole disk dedicated to OS X? if so, it's as simple as resizing the partition to make as much room for Ubuntu as you want. Make two partitions (one for Ubuntu and one the same size of your RAM for swap).
If using the live installer, when on the last page, choose the "advanced" box and choose to install the bootloader to the partition that Ubuntu is installed on. When you reboot, Chameleon should still be the active bootloader and will automatically detect Ubuntu and will offer the choice between the two OS's