Edit: Just realized I mis-read your post. I thought you were running PMS on the DS918+.
Info regarding file permissions still applies. Make sure your userid has R/W permissions for the directories.
You should also avoid the Synology created folders, as they can cause trouble for Plex.
Apologies for the initial mis-read.
What version of PMS are you running? It is listed in the DSM Package Center.
Alternately, https://app.plex.tv/desktop#!/settings/devices/all.
Using Plex Web, hover over one of the movies. Click on the vertical ellipsis in lower right corner and choose Get Info.
What is the full path displayed in the Media Info window?
This could be a permission problem. Your userid may not have enough permissions to see the folder contents. If not doing so, login with a userid with admin rights and give your userid R/W permissions for the folder. FAQ 1 provides info on changing permissions for the /Plex folder. Adjust accordingly for the video and music folder (full FAQ linked below).
Review Installation & Setup of Plex Media Server on Synology, especially Prerequisites and Pitfalls to avoid. Note the point about not using Synology’s pre-defined folders for Plex media. This will cause problems.
The Plex Synology FAQ also has much good information and is definitely worth a read.
