Why can't I set a metadata content provider on Plex?
Plex comes with 2 components:  The browser and the media manager or media server.  The media manager is what manages your movie library.  If you just go into Video Plugins and select Add Source, then Plex just treats it as a random collection of videos, not movies, and you cannot assign a metadata provider.

To add movies with the metadata providers, you have to add them to the Media Manager.  You access the media manager by clicking on the small Plex icon up by your system clock.