If you've made a reservation (room or package) you can add it immediately. If you haven't, you can once you book your package or room either online or on the phone with Disney. I personally think the sooner, the better.
I think it can be added no later then a week? Before check in. Someone can correct me please if I'm wrong. You can phone reservations to get a more accurate answer.
Thank-you all, I appreciate the replies. We've decided to put off the trip for a few months. Our son is getting married and we found today about about a few parties coming up for he and his fiancée.
We always wait until the deadline (72 hrs) gets close, because once we make it into a package it has to be paid in full within 45 days of check-in and the cancellation policy is more stricked than a room only ressie. Plus; me and my $$ do not like to part ways until we have too