With current instructions in ubuntu.com I know how to create an USB to boot from MAC. It works but it is not a persistent installation.
How can I make a persistent Ubuntu 13.4 Installation in a USB Key to work in MAC Book air?
Ubuntu – How to create a persistent USB Installation from MAC
macpersistenceusb
Best Answer
this is the solution for your problem...
http://studyblast.wordpress.com/2011/08/14/guide-mac-os-x-lion-how-to-boot-a-linux-live-system-from-a-usb-drive-how-to-update-any-ocz-ssds-firmware/
copy the file in a usb flash drive in fat32 create the folder efi inside efi create folder boot and inside efi/boot put 2 files the linux distro renamed boot.iso and the file bootx64.efi...reboot holding option and select usb device:done!