Go to www.waltdisneyworld.com. At the top click, "where to eat", then make dining reservations. On the new page, click on Dining Events. That will allow you to scroll down to the Tomorrowland Terrace Dessert party. Then to the right of that enter your date and number of people attending. (Enter the time 1 hour before wishes if you know it or just leave it at the default. Click Search for a table, it will search and let you know if there are availabilities.
Now one last question...each guest must have a ticket to the park - correct - or is there a fee charged for guest without a ticket and is only going for the Wishes Dessert party?