On Carnival the Past Guest program is automatic; no need to sign up. You can check on the carnival site in the "My Profile" section. There is a link at the bottom of THIS
Past Guest information page.
Your Past cruises should show up in your profile. If not your Past Guest number will be on your previous Sail & Card if you still have it. If not, just call them and they can get it for you, or any Carnival Travel Agent can as well.