If I am understanding the question, you will have to register your cruise where it says "My Cruise" at the top right of the main page. I am not sure if it will list the OBC's. If it does it should be in the Documents on the page that has the itemized charges listed. I think it is the first page of the "Guest Cruise Ticket Contract" section.
If that doesn't work, ask your TA or PVP to email a Guest Confirmation with the itemized charges/credits. I don't know how it works if you booked directly online with Carnival.