When you check in at the terminal, be sure to ask for the Daily Navigator. If you're on a Land / Sea vacation and you don't have to check in, ask for one when you board. The Navigator is the 'official' designator of when and where they will accept reservations for Palo and also for the Spa.
In general, on the Magic, they take reservations in Palo's on Deck 10 aft, usually starting at 1:30.
In general, on the Wonder, they take reservations at Wavebands on Deck 3 forward, starting at 1:30pm
Spa reservations (both ships) are taken in the Spa starting at 1:30 at Deck 9 forward.
Occasionally, they start taking reservations for Palo's 10 or 15 minutes before the stated time.