Turn off the Mac with the CD inside the machine.
Hold down the letter 'C' on your keyboard and turn the machine back on.
Keep the key held down until you see the MorphOS boot screen.
Failing that, you should check the ISO image isn't corrupt at a terminal in osx:
openssl md5 morphos-3.1.iso
It should return the value:
be6a161310f321c0d45e18b51244f4f9