I had a 2010 MBA and don't remember it allow Windows 7 to be installed via USB. You can download Bootcamp driver for Windows onto a USB but you still need the Windows DVD for the installation.
There are hacks to install Windows 7 via USB on motherboard designed for Windows, I don't think it works on Bootcamp.
The easiest solution is to buy or borrow a USB DVD drive for the MBA. You can use any USB DVD drives not just Apple super drive.