Ubuntu bootable USB drive does not show up as bootable
You don’t mention which Mac you have, OS Version or anything like that. It’s as if this question belonged on Ubuntu’s forums :)

In any case, I’m assuming you have some Mac computer and want to install or Boot an operating system other than Windows via BootCamp or the native OS X. In your example you need to boot from Ubuntu.

Turns out your Mac uses EFI instead of the archaic BIOS. So you need a little “helper” program in the boot process to tell the EFI to also look elsewhere. 

The steps for installing the tools and the process are perfectly described in the Ubuntu Community Wiki.

Before you go there, bear in mind the following things:


The instructions do not work for new iMacs.
The instructions are for CD booting (tho you have a link for USB at some point in the document).
You will need to install rEFIt, I did it on an old Macbook (White) and had zero problems, but the usual warning about “have a backup” applies.