As long as you have your booking number, here's how it works...
Go on the Carnival website and call up the Funship Dollars. On the address line put your booking number
On the next line put "Use as On Board Credit"
Then send it off. About an hour later you will receive an email confirming that it was received. Then you will receive another email stating it will be applied as onboard credit.
Approxaimtely 3 days later you can call up your reservation on Carnival's website, and click on Modify Booking. You will see your credit has been applied. I did this on Friday and the credit just showed up today, however the emails were both received on Friday night so I knew I was ok. I printed the emails off just to be safe and I will take them with me, but my cruise reservation now shows the credit has been applied.
Feel free to ask any questions, it sounds confusing but it's really easy. Much better than sending them back to Carnival, which I have done in the past because I didn't know this technique existed.