Since last year,
DVC members have not been able to do on-line 180+10 ADR's using the Disney website and their DVC reservation number. We had this same problem about this last year. I called 407-WDW-DINE and they were not able to override the system. They could see my reservation, but the system would not let them do anything about it. I had to call Member Services, who was able to manually make our reservations, which took forever! The only other option is to log-on and add your ADR's day-by-day until you get to 180 days prior to the last day of your stay.
It really stinks that DVC members are put at this disadvantage, particularly for those difficult to get dining experiences. There are several other threads on this topic, as you might imagine.