You cannot make Priority seating arrangements online (what a logistical nightmare)! To make those, you will need to either dial (407) WDw-DINE or wait until you get there and make them at Guest Relations, the front desk of your Disney owned resort, or dial dining from your phone in your Disney owned resort. That stated, since you are going during one of the busiest times of the year, I would suggest you make your Priority SEatings ASAP... They will start taking them 90 days in advance, with the exception of Spirit of Aloha and the Hoop De Doo Review, which book 2 years in advance.
Hope this helps!