Open start and type in "turn Windows features on or off" and open it. Make sure the one that says something like .NET Framework is checked. If 3DNUS is actually opening, it's probably installed. I remember this was happening to me too, but I forgot how to I fixed it. You should probably just...
Do you have the .net framework installed on your PC? If not, install it. If that doesn't work, here the CIAs for the USA 3DS:
New 3DS
Old 3DS/2DS
Edit: fixed wording
Edit 2: removed false warning
If you don't want to or can't update your system, you can use 3DNUS to download the official files for the needed app. There is a list of title IDs here. You need to add the beginning of the title ID to the "TIDLow".
I'll also put the title IDs here:
EDIT: I'm not quite sure about the O3DS...