I know this is a super-FAQ, but for some strange reason I cannot remember how to do this every time I need it. So, here it goes:
Simply reboot your Mac with the desired CD into the drive, and press and hold down, while rebooting, the Option key, aka ALT, aka the button between Ctrl key and the Command Key (aka Apple Key). You can now choose to boot from the CD.That’s all.
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The eternal fight between admins and computers 
Mac OS X keyboard shortcuts – Including startup shortcuts
Holding the option key on boot allows the user to choose a boot volume (any boot volume) from Apple’s Boot Manager. See the link at the bottom of the post for Apple’s support doc on this matter.
However, holding the ‘c’ key on boot specifically boots from the CD/DVD drive.
http://support.apple.com/kb/HT1310