[ubuntu] isolinux.bin missing macpro install issue

Hi all,
I'm trying to install Ubuntu 12.04 on a 2008 Macpro. I've created multiple boot disks following recommendations on this page. https://help.ubuntu.com/community/MacPro/ The computer refuses to boot the boot disks though. I'm using rEFIt, and every time I select the USB drive to boot I get the error below.

The catch is, the boot disks all start perfectly find on other computers. In fact I installed ubuntu on another computer with one no problem even after receiving the error from attempting to boot on the macpro.

Any advice would be great, thanks!

Edit: I'm created boot disks using unetbootin (which specifically says it won't boot on mac, but tried anyway), also the dd command for macs method, and using startup disk creator on a fresh Ubuntu 12.04 install. All of which started on other computers.