I've heard everything from calling after getting off the plane at MCO to five days, so I'm not sure what the answer is...if there is one answer. I've also heard one or two anecdotes of DVCers being able to add it on at checkin.
For regular DDP though, the problem may be that pesky "package" thing. It's a package, and if they don't have any package rooms available at this point, that could be your problem. Even though you've paid rack rate and bought a park ticket, if they don't have availability for that promotion, they don't have it. Hopefully, magic will occur and they'll be able to accommodate you.
Worst case scenario, you'll be at Disney this weekend and we'll all be sitting here wishing we were! Have a great trip.