It should be D or ctrl-d by default but I changed it a long time ago. Just go into file - subtitles - download subtitles (the shortcut is written next to that). You can enable a couple of other subtitle databases in the options menu.Wait what button is that? I just started watching elementary, but the sound mix on the torrent I got is shit, so I have been using plex to watch it for exactly that reason. But I only really have plex for my mobile devices and family, I would much prefer to watch it with mpc or vlc.
Thumbnails on how it looks on PC.