On your mac, there should be a file named /etc/yaboot.conf which is similar to the menu.lst or grub.cfg file in Linux/Grub. That is what you need to check and probably edit. Take a look at the site linked below for some information. It is written specifically for Ubuntu but the basics should be the same.

[all variants] PPC - Yaboot - How to configure the PPC Bootloader - Multibooting - Ubuntu Forums

If that doesn't help, google booting Linux from mac or something similar.