Are you using any DNS from the likes of Unblock-us, Unotelly etc on your router, and have static routes set up on your router to block Googles DNS?
If you do, disable the static routes and reboot the Roku and try again.
If not, reboot your router and the Roku, make sure its connected correctly to your home wifi, and try again.
The next step is to reset the Roku. On the side of the stick is a reset button, use a paper clip or if you have small enough fingers you might be able to press this in.
Press in an hold with power to the unit for approx 20 - 30 seconds or until you see the Roku welcome screen.
Then reconnect to your home wifi again, and try the activation again.
Another option you can try is to activate the device using a friends or neighbours wifi, if you get the same problems, the device might be faulty.