The booking number goes on the first address line, then "Use as On Board Credit" goes on the 2nd address line.
I got a confirmation email within an hour, and when I checked "Modify Booking" on the Carnival site, the credit was there within 3 days.
I booked my cruise through Carnival so that could be the difference. I had no problem whatsoever. I plan on doing this weekly until about 2 weeks before the cruise (I cruise 4/26). The first time I only did $25 which was the lowest amount, so that if something went wrong I wouldn't freak out so badly. HA.
I also odered and paid for my excursion on the Carnival site so that is all taken care of.
I got this information off of this site from another poster and I was thrilled that it worked. I hope you have no problems!