You need to log into My Trainz; the same place where you put in your username and password. You then find serial number on the left hand side. This opens up a new screen where you find your product and put in the serial number.
Once that's done, your product will be fully registered and it should appear next to your username. If you have problems doing this, I recommend clicking on the Helpdesk button found at the top of the forums and someone can assist you.