We called and let them know we would be adding it with our Reservation in Oct. We are going for 9 nights. That added it to our ressie....I think you have to add it at least 48 hours prior to your arrival. Same rules, everyone on the reservation (in the room) has to be on the plan for the same nights. My BIL is staying only 5 nights with us and he still had to purchase 9 nights of the plan. We actually pay for it when we check-in. Once you are 90 days out ( 180 after 10/27) you can go online with your DVC conf# and book your ADRs, which I highly recommend for the table service credits. Have fun!!
Becasue of BIL, all but one TS credit gets used before he leaves.
Currently it is 90 days, as of October 27, 2009 they are changing it back to 180 days...as well as the publishing of Park Hours 180 days in advance. So for June you can make you ressies in Jan instead of April.....that is awesome