If you want to book it, then call the hotel of Hilton reservations. I also checked on-line and got the same result as you, but when speaking to the hotel directly the reservations agent said he could book it for me, but in the end I didn't want to pay an extra $192 for the free extra night (we need 2 rooms for the NM clan). If it was indeed no additional cost over the 2 nights we booked, then I would have happily stayed the extra night, but alas that is not the case.