Its not clear from your question if hey told you they would give you a OBC or not. Did you actually talk to someone at Carnival?
If you have not talked to your travel agent or your PVP at Carnival then I suggest you do that.
Once you confirm you are being given an on board credit, then you can check it.
But how you do that depends on how you booked your cruise. If you booked through a travel agent, they can verify if a OBC is on your booking. If you use a Carnival personal vacation planner, thenyou should be able togo to the Carnival website and enter your booking number under "My Reservations" if you have not already done so. Once entered, go to that reservation and click on the "manage booking" one of the options it to make a payment etc etc.. there you should see a up to date billing summary with any OBC applied to your booking.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013