Apple powerbook G4 17" Trying to install Linux
I am pretty sure the Powerbooks don't boot from USB drives. You'll need to burn a CD.

I'm going to assume that you downloaded Ubuntu from the homepage at Ubuntu.com, which will give you ISOs for Intel (x86 and x64) processors. Since your computer has a PowerPC processor, you will need to download that ISO: Ubuntu's wiki for installing on PowerPC

Edit - Someone on the Ubuntu forums says you should be sure to download the livecd version and set arguments for loading the kernel:


  Be sure to down load the powerpc live disk. 8.04.1 will give you the least trouble with booting. On my Ibook I have to press the tab key as the first words come up as it begins to boot. This takes you to a screen with some options to load the kernel. The one that works for me is "linux-nosplash-powerpc".