Plex is very easy to work with; I use it to stream locally to 2 Rokus locally and to my PC remotely.
If you are having problems "seeing" the files via Plex, make sure they are in the right folder specified by Plex eg movies, TV shows, etc...
Second check the file names; there is specific nomenclature in regard to proper file names of the hosted titles.
Third, be sure to access plex on the host device (server) and be sure to hit the 'refresh media' button for the specific section.
Lastly, access the section, movies, TV shows, whatever and verify that the metadata is properly loaded and fix any mismatches.
Plex is usually easy as turning on a faucet or flipping a switch.