bootable disk for Mac Mini

[Tom Jenkins]

Good day,

I have tried twice to burn a CD from a downloaded ppc
version iso image that would boot on my Mac Mini. I
have inserted the OSX installation disk to make sure I
know how to boot from CD on this machine, and have
discovered that one does that by opening "System
Preferences"  and clicking "Startup Disk." All disks
available to boot into are listed in the window that
opens, then. When the OSX installation disk is in the
optical drive, it shows up as one of the options, when
the Ubuntu (or Mandriva, for that matter) installation
disk in in the drive, only the hard drive shows up.
I'd like to install Linux on this machine, and I'd
like to give Ubuntu a try, although I have been a
satisfied Mandrake/Mandriva user for several years, so
I'd appreciate any help I can get with this.

Regards,
Tom