Hi Leah. Corinne gave you a good link. You can also try http://www.birthmarks.com
They have a section about appeals, and even letters about the medical necessity of treating a PWS. I have heard from many people that they have to appeal before it is approved. Hang in there and fight them. It may not be a pretty road to take but in the long run, it is worth it.
Good luck and keep us posted.