I'm trying to install Ubuntu 10.04.4 on my Mid-2010 MacBook Pro 13-inch. I originally tried a simple CD install, but the computer is refusing to boot from any Ubuntu CD (I tried every version from 9.04 to 11.04, the verisons I have on hand). Now I used the Startup Disk Creator in Ubuntu to burn 10.04 to a USB drive, and whenever I select the USB drive in rEFIt, it just boots to the Windows 7 install. Does anyone have any idea what is going on?

(For the record, I have had several versions of Ubuntu on this machine before, so I'm at a loss as to why it worked then and not now.)