The MDR entrance for Any Time Dining is on Deck 2. (Sometimes if there is room, you may then be led to a table on Deck 3 which is for Fixed Dining. But you still enter on Deck 2.)
Only 50% of the Deck 2 tables are available to be reserved. This can be done by going to one of the podiums for Specialty Dining on embarkation day. They are normally near the Pinnacle Dining Room on Deck 2 and on Deck 9 between the Lido Pool and the entrance to the Spa. You could also reserve by calling Dining Services once onboard or talking with the MDR maître d. Some cruises will allow you to book a time only for the next three nights, some for the entire cruise, some only for that evening.