You can add the dining plan anytime up to 72 hours prior to your arrival date. If you add it within 45 days of arrival you will need to pay in full for it at the time you add it. If you add it within 2 weeks of arrival you may be paying a change fee of about $15. So the best time to add it is more than 2 weeks prior to arrival.
I see the Princess marathon is next week, so you will probably be paying that change fee, and you only have a few days left to add a dining plan.