Dual Booting a Macbook Pro (OSX) with Ubuntu 12.10
I had a sucessful live USB boot on a MacBook Pro mid 2012 with Ubuntu 13.04. I just followed the official tutorial.

Try uninstalling rEFIt. The live boot doesn't require it: just press alt at boot and select the live USB.

If you still have problems, I recommend you the rEFInd boot manager, which is currently developed (rEFIt is abandoned).

Before you install Ubuntu from the live CD/USB, I recommend to do the partitioning from Disk Utility.