HTPCmanager it's not meant to play/transcode or do anything with videos like plex does.
It's just an interface to manage other services like torrent downloaders (transmission, deluge. etc.), sickrage (tv shows downloads). couchpotato (movies) and so on.
I also use Kodi to play all my stuff, nice interface, runs great on a raspberry, play all my files directly from my nas via smb, no transcoding needed.
For people that want an alternative to Plex, you might want to check out Emby, it's basically the same as plex but free both server and clients.
There´s even a plugin for Kodi that lets you use the Emby server library but using Kodi player and interface, pretty neat stuff.
However, I don't think we will be able to run Emby on our DLink nases because it requires mono (which is not available as an addon yet), and it is a bit heavy, at least on the ram side. But running it from a windows/linux server works fine.
I think now it´s even available for the raspberry pi 2, however I don't have one yet so I can't tell for sure.