You can go ahead and use your interface cable. Just go to Ross-Tech.com and download the latest version and install it on your laptop. The first time you use it it will demand that you test it. Plug the cable into the computer and into the car. Turn on the ignition (you do not have to start the car), and start VCDS program. Press the options button, make sure USB is checked and press the "test" button. It should find your interface, press "OK". With the latest software, it might tell you that the interface's firmware needs to be updated, again click OK and let it happen. After this just press "save" to save your configuration and VCDS is ready to go on any car.
Your cable is already ready to go. The only thing that "registration" is required for is direct support from Ross-Tech if you have any problems. If you bought it from an official vendor, you are registered. The "license" to use the software is in the cable, you don't need anything else from Ross-Tech. Enjoy your new tool (toy?).
Have Fun!
Don