The RealSense cameras do not require a product serial number. Instead, you can get a serial number by registering your personal details (name, email address, etc) when downloading the RealSense SDK.
If you are trying to download the '2016 R2' SDK, you can skip the registration and launch the SDK download immediately using this link:
Please know that the latest '2016 R3' SDK will not work with the R200 camera. It will only work with the F200 or SR300 camera.
If you still want to download the 2016 R3 'Essentials' core module, you can do so without a serial number by using this link: