Can't create bootable USB

Now, I know this is the most basic part and I have actually done this many times before. I've created bootable USB sticks for Fedora recently (using the same USB stick) and in the past for Ubuntu using the instructions here http://www.ubuntu.com/download/deskt...ick-on-mac-osx, but for some reason it doesn't seem to be working for ubuntu 13.04. I am using sudo dd to create the disk but as soon as it's finished I get a warning saying "this disk is unreadable", and it doesn't boot.

Unetbootin doesn't create a disk that is bootable on a mac and I have no optical drive so this is my only option. Any ideas what I might be doing wrong?

Thanks in advance