I have an AP voucher and was able to access the site. I put my numbers in on the last choice... I think there are four areas for different APs and vouchers, but they're not clear on which is which and what numbers go where...

Just play around with it. I wanted to see for myself what was available before I called to book. I read (I believe it was on Mousesavers) NOT to book on the Passsholders website, because the cancellation policy is different than if you book by phone, and not in a good way!