So you have an old USB HDD enclosure and a plain ol' DVD drive, but what you really need is a shiny external DVD Drive. Today you're in luck, because our friends at Lifehacker have instructions for a DIY frankendrive.
This is a neat way to use parts that you've got sitting around, but if you don't already have them all I'd recommend just plain buying an external drive. Unless you're really into DIY projects, that is. [UberModder via Hack a Day via Lifehacker]