The luggage tag you use is included in your cruise documents, or edocs, as Pete noted. Your edocs may not be ready yet. When you access your booking online, you see two items on the right....one says "Online Check-in" and the one below it says "Cruise Documentation". If the status for Cruise Documentation says "not issued" then you will need to wait. When it says 'ready' or something like that, you can access it and you will see a detailed luggage tag to print, with your cabin number and deck on it.
Do not use the sample tag you get with your seapass, which is what you completed under the "Online Check-in" window.