If by addons you actually mean just the Plex for Kodi add-on then it (the dl sub function) seems to not be functioning at the moment. It (the dl sub function) also doesn’t work in Kodi on Windows either. This is something you would need to take up on the Plex forum as it is their add-on. As a workaround you can use the Plex web client and in the details screen for a video item there is a section for subs and it has a option to download there. This would then make that sub available to Kodi.
I did a quick test with Composite and that was able to download a sub just fine using opensubtitles during playback. I would expect PKC to be working as well. Do be aware that the opensubtitles add-on requires you make an account on their web site and to manually enter your credentials in the add-on’s settings page before it will actually work.
(edited for clarity)