As long as you already have the Disney Cruise Line app, all you need to do is open the app while in the cruise terminal (or onboard the ship) and select the "On Board or at the Port" option from the main screen. You'll need to enter your cabin number and birthdate, and that should take you to the onboard Navigator version of the app.
Once you're connected to the wifi if you already have the app you just select "on board" and it'll walk you through the steps asking for like room number and such.
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