I believe you can make them on-line, but it's best to call Ships Services at 1-800-541-1576.
I don't believe that you can make a reservation for a particular date - that you will have to do that at the Pinnacle Grille once you get on board.
I just found this:
Q. Can I pre-select specific dates and times for a Pinnacle Grill reservation?
A. No, the guest for whom you purchase a Pinnacle Grill dining experience will receive a voucher delivered to their stateroom on the day of embarkation. They will use this voucher to make a Pinnacle Grill reservation for a specific date and time once onboard. You can request a specific date and time which the ship will try to accommodate, however you or the recipient guest must confirm the reservation with the Pinnacle Grill once onboard. This also applies to couples packages that include a Pinnacle Grill dining experience. For parties of 6 or more guests, a special form must be filled out prior to the reservation being processed. Please call our Ships Services department at 1-800-541-1576 to make a reservation for a party of 6 or more guests.
It's under "For Booked Guests", FAQ