You have to request it through the senior cruise director for Carnival, John Heald. Go to his blog, and go to "ask John" and he'll answer you. You'll get an email from the Fantasy telling you you've been confirmed.
Tell him when you are sailing, and how many you will need.
Carnival Fantasy '13 / Enchantment of the Seas '13 / Allure of the Seas '13 / Carnival Ecstasy '12 / Carnival Freedom '12/Carnival Imagination '11/ Oasis of the Seas '10 / Norweigan Magesty '08/ Sovreign of the Seas '07/Carnival Inspiration '06/ Carnival Miracle '05/ Carnival Inspiration '04