Welcome to the Apple Support Communities.
1. Boot Camp 3.1 is only available as a Boot Camp driver update. If you haven't installed Boot Camp drivers yet, insert the Snow Leopard disc and install the drivers on Windows. Then, open Apple Software Update on Windows, too, to install Boot Camp drivers 3.1, and then, 3.3.
2. On Snow Leopard, the Windows support software is only available as a download for the Late 2010 MacBook Air. For the rest of the computers, you have to install it from the Snow Leopard disc or the Mac OS X DVD that came with your computer