You either need to register, and pay a monthly service for the new box or take one of the existing boxes from your house. It is no different than adding an additional box to your regular account.
Just because you have the dome antenna does not mean that it is a simple as plug and play. For my in-motion dome I had to go through the set-up and designate the type of antenna I had, which turned out to be the closest match which was an 18" single LNB. This works for my Kingdome. When we want to watch HD I then have to reprogram the DirecTV box to recognize my Slimline 3LNB antenna. As I said I guess it depends on the type of box that you have but if the box takes a card, I doubt if it is the newest one available.