I had the same problem. The key seems to be to only have one weapons folder (I managed to have two somehow, one in the root and one in objects). I deleted the one in the root, and used the weaponeditor to edit the weapondata.ini that was in the weapons folder of objects. I think I also deleted a weapondata.ini that was in the objects folder - I looked at SFP1 to see what the setup was there, and there was only one weapondata.ini (I had three at one point!). After that, they showed up on the Nieuport.
I'm at work so I'm remembering what I did last night, but I'm pretty sure that was the sequenece. I also did the "drop the weaponeditor.exe in system32 folder of windows and it worked fine. I used the one from the latest weapon pack. All I did was launch the editor, click on open, select the weapondata.ini, click on merge, select the add to weapondata.ini file, click on save, and voila.