When I try using the emulator (interface) from the dlink download webpage, I can see the different screens and browse for the updates, but I get a message saying that the update function is not active on that version of the emulator.
This is because the emulator is just emulating the UI experience and does not have anything functional behind. It also runs entirely on D-Link's webserver, meaning that it has no connection to anyone's real NAS hardware. And you can't download it from D-Link's website. It's there just for you to see how it should look on your real device, probably aiding you in your purchase decisions.
If the Web-UI on your NAS does not act like expected, try a different browser for starters.
If your FW file is correctly unpacked, you shall see the strings
DNS-325 and
Linux-2.6.22.18 at the beginning of the file if you open the file in a text editor:
If you don't then your file is in wrong format for upload into NAS.
PS you also might want to verify the length of your FW file:
dns325_FW_101 26 278 308 bytes (26,3 MB on disk)
dns325_FW_102 27 250 440 bytes (27,3 MB on disk)
dns325_FW_103 30 012 181 bytes (30 MB on disk)