HBO Go activate is for people who already subscribe to the channel on cable or satellite and want the option of using it on their computer, set-top box, or mobile devices. You have to have a subscription and an account with your supported TV provider. HBO Go does not include every cable operator in the US.

HBO Now is a paid service that you can sign up for $14.95 per month independently of a cable connection. You don't need a subscription to HBO to get it and you have access to everything you'd get on HBO Go.

HBO go activation | HBO activate