The Relay Station might not be connected to power (Check to see if there is a green LED on the top of the Relay Station)
The Relay Station might not be connected to the internet.
The Relay Station could already be registered to an account. (Check to see if there is a green and red constant LED lit up on the top of the Relay Station. If not, then hold down the button on the back of the Relay Station for at least 35 seconds before letting go. The green and red constant LED should then light up, and you should be able to register it)
The Relay ID could be invalid (check the label again to make sure you entered it correctly)
The Relay Station could be in an error state (try rebooting the power to the Relay Station by unplugging the power adapter for 10 seconds and plugging it back in)
The home router may need to be rebooted for it to recognize the Relay Station. Try removing power to the router for 10 seconds, and then replace the power. After one minute, try re-entering the Relay ID.
Sometimes, ports may go out on routers. Try plugging the ethernet cord from the Relay Station to each of the ports on the router.