Are you having a trouble to boot a linux installer from USB flash on mac machine ? and you already tried this instruction but still no luck http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx
lately i have been googled around this topic but none of those instruction worked.
Finally i have found a simple tool to create linux bootable usb on mac which is very easy to approach.
. Go to download Linux USB Creator for mac http://penguintosh.com/2012/02/26/linux-usb-creator-released/
. Launch the tool and follow the screen instruction or you can follow the instruction in the website. after that reboot your mac and press the alt key to boot from your USB flash.