SPK file for Plex to your Synology server. You can use the Manual Install option (as seen in the top picture) then visit and manually upload the. Here's the gotcha, and why I'm writing this up for you.

So the client is saying 'you need version x+2' and all I see on the Synology feed is version x. The issue for me happens when I go to the Synology to update and there's no new update. This means if you aren't keeping your Plex Server reasonably up to date, one day you'll startup your iPad Plex app or Windows Plex app and be told that server version is too old. If I can be slightly critical of Plex for a moment, they are fairly aggressive about keeping their client and server protocol versions in line. However, I personally (and others) have found this feed of packages to be either updated not very often, or when it is updated, versions don't line up. Synology has an ersatz App Store that allows you to easily install all sorts of stuff to your Synology NAS, like phpBB, Plex, Git, and more. I also run CrashPlan headless on my Synology. I have a Synology 1511+ NAS and run everything on it from Minecraft Servers to Plex to using it as a giant 5TB NAS to storing my Time Machine backups and Windows 8 File History. Last year, Plex came to the Synology DiskStation. Best yet, the Plex Server can run not only on your spare computers, but also NAS (Network Attached Storage) devices like Synology, Netgear, Drobo, and more. Plex has clients for iOS, Windows Store, Windows Phone, and on and on. Plex is a server you run combined with both a great HTML5 web app and some awesome native clients.

Plex microsoft store