Probably a video covering DSM 6. Synology forced a lot of changes with DSM 7. Among other things, the user plex is now PlexMediaServer and is a System Internal User, not a regular user account.
Additional Info:
DSM 7 - Plex Media Server - Release Info & Migration instructions
DSM 7 - Packaging 2.0 (PMS 1.24.2.4973) - Release information
