[Files don't actually need to have extensions, and folders can have them too, but...]
I was referring to neither a folder nor a file, but a
registry key, a shortened form of:
Code:
Computer\HKEY_CURRENT_USER\Software\Firaxis\Nexus\Viewer
Note that it can be dangerous to edit the Registry if you don't know what you're doing (i.e., you can really mess up your Windows installation), but that seems to be your only recourse. Run regedit.exe, and navigate to the key above. You should see two String values in that key, "AssetPath" and "Executable." The AssetPath should be to your CiV installation folder. If you delete the AssetPath key, it should cause Nexus to try to figure it out again, either by reading other Registry keys or by prompting you. You can also try putting the path in manually. Make sure you're editing the value's data, not renaming it from "AssetPath".
That refers to opening the Nexus program, clicking on the fourth tab labelled Nexus, clicking the Script Browser button on that tab, selecting "Extract PAK (*.fpk) files", then clicking Run Script.
As far as the error message, I think there may be a few different reasons for that one. You want to make sure that another registry key (HKCU\Software\Firaxis\Nexus\Civ5Settings) has a String value called "GameBuild" pointing to the full path for your CivilizationV.exe. If that's correct, then there could be a permissions issue (try running Nexus as administrator).