Just go to Carnival.com and click onto gifts, pick one as though you were going to purchase and it will ask you for your booking number and then your cabin number. You should know the category you booked. With a layout of the cabins, you start entering cabin numbers within your category. You will know when you hit the right cabin bacause it come back with your sailing date and your name. It can take some time to find the right cabin. If you were booked with a group rate, you may not be assigned a cabin.
We are going in October, on the Spirit and I located our cabin but my Sister who booked an inside cabin usuing the TA's group rate has not been assigned a cabin.