Plex Media Server version?
Synology model?
DSM 6 or DSM 7?
If DSM 7, read these two documents. DSM 7 is very different from DSM 6. This forced many changes in how Plex is installed.
DSM 7 - Plex Media Server - Release Info & Migration instructions
DSM 7 - Packaging 2.0 (PMS 1.24.2.4973) - Release information
