Ok its totally confused me too.
You can book ADRs in advance if you are on or off site.
ON SITE. They will only take bookings 90 days from your check in date and you can only book up to 10 days after this so if you are staying for 14 nights you can book for the first 10 and then call back four days later for the others. I had no problem doing this when I booked mine last month.
If you are OFF SITE then you can only book one day at a time which means lots of calls or as some do work out what date your 90 day + would be and then book them then so 90 days out work out what date would then be 14 days after and you can call and make them all then.
You can now also make reservations online but as I did not use this someone else that has can give you advice.
You DO NOT have to be on the dinning plan to book ADRs most of the signature restaurants will require you to pay a deposit to secure your reservations (think Ohana is one of them but we have not booked any of those maybe some one can give you a list) We are on the Dinning plan but its the counter service one but we are going to treat ourselves to a couple of Table service meals and pay as we go.
Hope that helps