We did this on our trip last December. Once you check in (and you have to be checked in ) go to the guest services desk and ask about availability. We were staying at VWL for 11 nights so we have several choices. We took the first opening which was 4 days after our arrival. We met the ranger at guest services at 8 am on our day. It so really cool. What a view from that rooftop!! Make sure you bring your camera.
You can actually ask the CM about it while you are checking in, no need for a separate trip to Guest Svcs. Do it then, we were able to get day 6 of our 7 day stay -- and that was our only choice.