As far as I know there is no registraton process for the Google Wifi, If you bought it and you keep you receipt, I guess it is still on warranty.
If you want to read the small print I found this site:
I just wanted to jump in real fast to see if you saw Martini's reply and to see if you still needed some help on this or if you were able to get it sorted out. If you are still needing some help, just let us know and we'll be happy to continue helping.
Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, I would be happy to keep it open. If there's more we can do, just let me know.