Note that multiple operating systems booting on a single machine is still something of a black art. This document does not even attempt to document the various boot managers, which vary by architecture and even by subarchitecture. You should see your boot manager's documentation for more information.
For Macintosh, Atari, and Amiga systems, the original OS must be maintained on the box and used to boot GNU/Linux. This step is not possible for these architectures. It is essential that, when booting MacOS in preparation for booting the Penguin linux loader, you must hold the shift key down to prevent extensions from loading. (If you don't use MacOS except for loading linux, you can accomplish the same thing by removing all extensions and control panels from the Mac's System Folder. Otherwise extensions may be left running and cause random problems with the running linux kernel.