1st download the Lubuntu 13.04 ISO file PowerPC http://cdimage.ubuntu.com/lubuntu/releases/13.04/release/lubuntu-13.04-desktop-powerpc.iso

2nd Burn it with Toast on Mac of Nero/Infrarecorder on Windows.

3rd Boot the CD on your MAC. You have to hold the "c" key while the PowerMac is booting.

4th After booting, click on "Install Lubuntu" icon on the desktop

5th Enjoy