If you return to the download page, then you should see details about registering your software. I'm guessing that you need to do that if you want to use that update, is that correct?
If your Cubase software is presently not working, you could also try reinstalling Cubase from your original disks. The serial number should be with those, or, you may then have working software, and then you may have access to a serial number in an 'About' box.
Maybe...