This can be easily done using hdiutil command tool. The file you downloaded is in ISO format but we need it to be in IMG format. Ubuntu 14.04 is the latest LTS version, and this is what I would recommend to you. Since you are using a Macbook Air, I suggest you to download the 64 Bit version of whichever version you want. Jump to Ubuntu website to download your favorite Ubuntu desktop OS.
Of course, you need to download ISO image of Ubuntu desktop. Of course hit the Partition button to format the USB drive. It will give you a warning message about formatting the USB drive. When all is set to go, just hit the Apply button.
