Carnival has Club O2 on every ship. The only difference is that one some ships they actually have their own Club Room.
I know the Fantasy class ships have their own Club Room.
Regardless, attend the meeting the very first night listed in the Capers to sign up. If they don't have their own Club Room, they simply meet at different places around the ship. They will give you a Club O2 Caper that will give you the activities, time and location for the entire cruise.