I know nothing about the boot process for a Mac but in a PC you have options
in the BIOS setup. This is (usually) reached by pressing DEL or some other
key when the first screen appears on boot up. These options allow you to
specify which device it boots from, which could be the CD drive, floppy
drive or hard drive. The default is probably not the CD drive. Try to find
this information for the Mac and see how to set it up to boot from the CD.
If you can do that, and you have the Ubuntu CD that works with your CPU etc.
then you should be able to start and install Ubuntu.