All you have to do is ask, but it's very popular and the openings fill fast. While you're checking in, ask if there are any spots available. If there is an available date, they'll sign you up for it. The night before you're scheduled, they will slip a note under your door to remind you and to tell you what time to be in the lobby.
We did this last December and it was really a neat experience. We had to be in the lobby by 8am. They took us up to the 7th floor and then you walk up several flights of stairs to get to the roof. You help raise the flags and they give you quite a bit of time to take in the wonderful view from the rooftop and take pictures. They'll also give you a certificate that's a neat keepsake.
Just be sure and remember to check for availability while you're checking in!