Speaking from experience on this matter.. You can get the DCS-6511 to work on the DNS-726-4.
I see on their website they still have just firmware rev 1.20..
Basically just select a different DCS- camera and it will come up.. I forgot which model number
it shares the same SDK with... Try the DCS-6111/6110 or one of the other recent cameras on the list
and it'll work.. I had it working about a year ago.