I called carnival and they gave it to me after some VIP treatment. I guess they wanted to make sure I was who i really am. It also appears on the currents magazine you receive from Carnival as a past guest.
I emailed carnival from their website like 2 years ago to get mine. Cause I was on like 3 Carnival cruises and never got a past guest number. But I emailed them and told them the ships I've been on and the dates, and cabin numbers. But last I knew trying to Contact them from their website wasn't working. So I'd call them if I were you.