There's a bunch of places online that detail how to make bootable USB drives for Linux distros like Ubuntu on Mac. I can't post a URL, because it says I need to have at least 5 posts. It seems to be...
When I create a bootable Linux distro installation USB drive, I use this command: sudo dd if=/Path/to/linux_distro.iso of=/dev/rdisk<disk number>
bs=<number of bytes>