Technically yes, it must be activated so that you have a card with your name on it. You just need to stop by the guest services located at the entrace to all of the theme parks to have it exchanged. You could always try to register at your hotel without it, then explain you haven't had a chance to get it activated yet. I would think they would just have you stop back by to show your card later, or not even ask for your card at all. It is up to whoever is working the front desk at the time. I have always had mine already activated, but I have never been asked to show it either. I guess it would be up to you and what you want to do.
Laura