How to use the Live-CD with a Mac
From MEPIS Documentation Wiki
To boot an Intel powered Macintosh Computer like a recent model ibook or PowerPC with a SimplyMEPIS Live-CD, insert the Live-CD into the optical drive and reboot the computer.
As the Mac powers up, hold the "C" key to force it to look for boot code from the Optical drive first. When you see the Grub boot screen, you can proceed as usual.