So, I actually IM'd about this, and they told me that you had to have a reservation made first, and then submit your wait list request. If you submit your waitlist request first, the check box to replace existing reservation doesn't show up. Being a new owner, I never used the waitlist, so I wasn't sure if I should reserve first or just trust that I would get the waitlist. After reading the message boards i figured I should probably back up my waitlist request with an actual reservation.