In Mac OS or OS X, how can I boot from a CD or DVD and/or ignore my startup disk?

Normally, to set an alternate startup volume, such as a bootable CD or DVD, you should use the Startup Disk system preference or control panel. However, if your regular boot disk is damaged or if you can't set the CD or DVD as a startup device from the system preference or control panel, try one of the following options:

• Restart your computer and immediately press the Option key. Icons for all available startup volumes will appear. Click the one you want to boot from, and then click the right arrow button to complete the startup process.
  
  
• Restart your computer and immediately press Cmd-Option-Shift-Delete. You must press all the keys at once. The computer will start to boot from the CD or DVD drive. If there isn't a bootable disc inside the drive when you begin the reboot, the computer will attempt to boot from another partition or drive.
  
  
• Restart your computer and immediately press the  c  key. The computer will look for a bootable CD or DVD and, if it finds one, will use it as its startup device.

Also see:

• On my Mac, what do I do when a disk icon with a question mark is blinking on my screen?