I don't remember that option either when I was making my booking for this summer for the carnival victory that's why I am asking...
Cas, they didn't add the option until late November. It should be available now for your booking.
On the page in this link
click on the "Your Time Dining" link. It is under the picture and the the "Delicious Choices". It will give some info along with the ships it is available on.
They left out the Destiny, Imagination, and the Inspiration. However they were supposed to implement it on those ships last year. So, I'm not sure if they just inadvertently left them out or they were delayed yet.
The implementation schedule for the remaining ships is;
This will be an option as well on the Carnival Magic.