Using the "check for upgrade" or whatever it says button within the Router firmware also told me that there was not a newer version available. That was after 1.33NA was available non-Beta and I was on 1.32NA.
Just go to the dlink site and browse to a newer version for the router, then download it and follow the instructions.
The router firmware would not find it - I guess ANOTHER bug in the firmware, at least in the 1.32NA firmware.