You will find your code on the sleeve of your Saffire Drivers Resources CD. This is included with Saffire. The registration code is used to authorise your host VST & AudioUnit plugins to run natively on your Mac or PC.
When you run the Plugin Authoriser, choose internet activation. You will be forwarded to the activation web page in your browser. Enter your registration code on this page in the field labelled as such. For more information, try the Getting Started Guide.