First, quit all applications that are using files on the disc. Then try one of these alternate methods:
Choose Apple menu > Restart and hold the mouse key down until the disc ejects.
Use Open Firmware to eject the disc if all other methods fail (this may not work on all computers). To use this method, you have to start up and briefly use the computer using Open Firmware, not the standard Mac OS X desktop. This is a simple procedure-try it if you're feeling adventurous. (Instructions in your help)