i have followed the instructions to the letter.and i am computer literate lol.but it seems mac does not want me to do this! i did install REFIT,set a partition for ubuntu.downloaded and burned the 12.04 version but it gets to select which cd mode 1 or 2.then my keyboard locks up.should i try the 13 version?.my mac is an older model 2008.I have tried using ubuntu,xubuntu and mandriva.
I had the same problem booting the CD on an older Mac Pro, about 2 years ago. You've done nothing wrong; the problem has to do with the older Mac hardware. I was told that there is a separate "+mac" distribution that will boot on these older machines (something about an EFI conflict?). I found one, and that worked for me. You may have to Google/search for such a distribution. Good Luck!
EDIT: Try the Alternate Install CD for Macs at Ubuntu 12.04.3 LTS (Precise Pangolin)