If you have an mmsi number, it is already registered. It's unique to the boat and carries a packet of information that you can create or edit in the app's settings section.
So... it's just a matter of finding the "settings" icon on your app's display, then look for the section concerning boat information.
On the boat beacon app above, the gear icon on lower right corner of display is for settings. Once opened the heading "boat details" will take you to the right place.
I took a look at the "vessel finder" app on its website (sorry but it seemed lacking compared to the two below)... and I must say that you might be happier with "boat beacon" or "Ship Finder". The full app is only $9.95 and works flawlessly.