Once you've installed DD-WRT for the first time, you just need to use the revb-ddwrt version of new builds to upgrade from within DD-WRT's admin pages, and not the factory version. I don't know what's different, but these are the instructions from the creators. It seems you use the factory file when upgrading to DD-WRT from D-Link's own firmware or when installing it for the first time via the Emergency Room.
http://www.dd-wrt.com/wiki/index.php/DIR-600