What available methods are there to boot a Macbook Pro
I would follow these steps to get Ubuntu on a freshly erased USB drive.


http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx


No need for rEFIt or other hybrid boot tools so you can get your feet wet booting with the option key held down and choosing the USB drive. Once you have a working linux, you can easily bootstrap to other OS if they don't have instructions that are as Mac friendly as Ubuntu.