Trying to dual boot Ubuntu 12.04 on Mac from DVD but rEFIt only gives "boot from osx" option. How to fix?

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http://askubuntu.com – I'm trying to dual boot Ubuntu 12.04 on my Macbook(which is a Mid 2007 model with Intel Core 2 dual and currently running Lion). I've burned the Ubuntu desktop iso file to a 4.7 dvd, installed rEFIt, and partitioned my hard disk to make room for Ubuntu, but when I put in the dvd and then reboot, the rEFIt menu shows only the "boot from Mac OS X" option instead of asking me whether to boot from CD. Holding down the key "C" or the option key doesn't help either. Could you explain how to fix this problem and get the laptop to boot from the DVD properly? Thank you very much! Eddy (HowTos)