[ubuntu] Installing ubuntu over mac os x

I've recently come by an old Macbook Pro which i want to change to run Ubuntu. First of, I'll start by saying the CD-drive does not work so I have been trying to boot it off of a USB. It has not worked though. I followed the guide on this page: http://www.ubuntu.com/download/help/...ick-on-mac-osx. This mac runs version 10.4.11 of Mac OS X an is 3,1 model type. I download version 10.10 of Ubuntu and mounted it onto the USB "properly" (to the extent of my knowledge) but it won't come up in the boot options when i startup the comp. and hold alt/option. It also does not show up in the startup disk menu.

Any help would be greatly appreciated.
Thanks for you time.