Unlike the other guides, I suggest selecting Install Ubuntu from this menu. Next, GRUB, another bootloader, will appear as white text over a black background (like DOS) and give you options to Try Ubuntu or Install Ubuntu.Choose the first Ubuntu icon with the arrow keys on the keyboard, press Enter. rEFIt will give you the option to boot Mac OS X (Apple icon) or Ubuntu (this might appear as two separate icons depicting four squares in a diamond configuration). Reboot your MacBook Pro with the Ubuntu USB drive inserted.When it is all done, Mac OS X will not recognize the disk and ask you to initialize it. It will involve downloading the Ubuntu 13.04 ISO image, converting it for Mac OS X, and using terminal commands to write the converted image to your USB drive. Create a bootable USB Ubuntu disk with this guide for Mac OS X.It will have your Mac OS X installation highlighted. Reboot your Mac twice and you should see the rEFIt bootloader screen appear after the second reboot. Boot into Mac OS X, download the rEFIt bootloader, and install it in Mac OS X.Install Mac OS X on the Mac OS Extended (Journaled) partition.Format the first partition as Mac OS Extended (Journaled) and the second as free space. Using a Mac OS X 10.8 bootable USB flash drive (create your own by following the DIY instructions here), partition your drive into two equal partitions with Disk Utility.The following is the process that I used for successfully having Mac OS X and Ubuntu play well together on my MacBook Pro Retina (MacBookPro10,1) (15.4″/2.6 Quad-core i7/8GB/512 GB SSD) However, I ran into a problem with Mac OS X 10.8 Mountain Lion not waking from sleep due to using the rEFInd bootloader (more information about this problem documented on this thread).

There are a number of useful guides to installing Mac OS X and Ubuntu in a dual boot configuration on Macintosh hardware such as James Jesudason’s guide here or Alex Victor Chan’s guide here.

Mac os 10.8 installer apple