This page is about installing Ubuntu on a MacBook Air, using the Ubuntu Desktop CD and the Apple External Drive, which needs to be purchased separately. When using the external drive, make sure you connect the power cable to your computer.
Note that the Macbook Air Superdrive is the only external drive that you can use to boot into Ubuntu from a CD. Intel Macs will boot Mac OSX from any USB device under certain conditions, but not other operating systems. The only exception to this rule is the Macbook Air Superdrive external DVD drive. There have been some reports of success without the Superdrive, by making a small separate boot partition on the Macbook Air disk, and booting into Ubuntu from that - see http://www.grimsby.us/3/ for an example, but others have tried and given up - see http://castyour.net/node/74