You should install the current released version of Plex Media Server.
Scroll to the bottom of this page. Click on Plex Media Server. Download the package for your NAS, Intel 64-bit, DSM 7 version.
Perform a manual installation using Package Center. Click the Manual Install button, then choose the file you downloaded.
After installation completes, use a incognito/private browser window pointed directly at the Plex server: http://syno_ip_address:32400/web, substituting the real IP address of your NAS.
Due to a quirk in DSM 7, you have to use the direct method for initial setup. You cannot click on the “Open” button in Package Center, use app.plex.tv, etc.
Once you have claimed the server, you can then use app.plex.tv, etc to manage the system.
These documents will help:
DSM 7 - Plex Media Server - Release Info & Migration instructions
DSM 7 - Packaging 2.0 (PMS 1.24.2.4973) - Release information
