Apple powerbook G4 17" Trying to install Linux

I believe followed the directions on how to create a bootable USB drive from the Ubuntu website. My question is how can I get my Powerbook to boot from the USB drive? I tried doing the command+control+o+f and it just shows me the file icon with the question mark I also tried booting it holding the option button and it directs me to a screen with two buttons a refresh and a next button, I click on the next button and nothing happens?? kinda stumped.