If you will ask Apple or consult any user guide for the MacBook air it will say that the notebook has only 1 USB port. Is that true ? not really. If you use the developer utility “About This Mac” you will notice there are 7 buss-es, and only some of them are being used.
If you plug in an external device on the only known USB port, it’ll show up on the same bus as the iSight, so this is where you get the idea that if more buss-es exist without anything connected to them, you could use them to create a new USB port.
The whole process of adding a new USB port is very well illustrated but since fine soldering will be used on the MacBook’s motherboard i would not recommend this project to a beginner. Its better to be stuck with a single USB port than to mess-up your MackBook AIR.
MacBook Air hack to add additional USB ports: [Link]