Question: how to install plex on synology?
Answer: Download the Plex Server package. (Choose "Synology" under "NAS".)
Connect to your Synology NAS.
Open up the Package Center. Source: Windows Central.
Click on "Manual Install."
Click "Browse" and locate the Plex Server download.
Click "Next." ... 
Click "Yes" on the pop-up.
Click "Apply."