As far as I can work out they open the reservations much later. I can't do the link, but there is a thread on the main trip planning / dining reservations board called " Wishes desert party thread #2 - March 2013 open". Where one of the first posts gives the dates for the last few months. I'm still waiting and watching for April. If you subscribe to that thread hopefully you will find out when they open it up for your dates ( that's what I'm hoping) Good luck
From what I remember from when we booked it, we had to call the reservations line to book it. As PP said, they open up reservations for the Dessert Party much later, it's not 180 days if I remember correctly, I think it's 90 days. My advice would be to call the reservations line and ask them if you can book and if it's not open yet, ask them when it will be open for reservations.