I don't recall Disney sending us actual registration forms; we used the online check-in (you can do it 75 or fewer days before your cruise date) to complete the forms. It was easy, things like passport number, emergency contact, read and agree to the cruise contract, flight information if any, things like that.
If you don't finish them, or forget, or don't use the internet, doesn't matter. You can show up at the cruise terminal with (1) Passports (or birth certificates for some cruises if you so choose) and (2) payment method (credit card) and they will have the forms there you can fill out on the spot. Doesn't take long. You can also make reservations for excursions and adult dining online, if you want to do any of those.
Checking in online does let you choose a port arrival time and does save you some time in the terminal.
Don't stress, it is easy. And remember the cruise "documents" (luggage tags, review of your reservation, advice on how to spend more money on the cruise LOL) won't be arriving until shortly before the cruise. If you are ever worried that they haven't shown up, just give Disney a call for their reassurance. But they will show up, have faith!