This procedure is intended for certain older Macintosh computers, and does not apply to the iMac and Power Macintosh G3 (Blue and White). For these, and later, Macintosh computers, the recommended method of starting up from the CD is to hold down the C key during startup.
Older Macintosh computers that did not support the C key as a method of starting up from the CD can use the Command-Option-Shift-Delete key sequence.