Platform Builder is purchased though a vendor who sells it and CE licenses. A link to those vendors is available here. That said, you probably don't need to to just uild a driver - eVC works fine for creating drivers. There's a starting point for your webcam driver here.
Thanks for the info. Actually, I tried building that project but do not have usbclient.h nor usbclient.lib. Where can I get those from? I think if I have all the includes and libs then that webcam driver should built no problem.
If they're not in the ADS SDK, you can ask them for it. The other option is to just order an eval version of Platform Builder, though it may be difficult to find 4.1.