How do I install VLC extensions?

To install extensions (.lua) the needed folder is ~/.local/share/vlc/lua/extensions/ for the current user and /usr/lib/vlc/lua/extensions/ for all users. I found about it here and here.

Once installed, they are to be found under 'View'

(I put them here for illustration, I do not recommend all of them.)

Skins are installed by going to Tools/Preferences(Ctrl-P) - Interface: 'Use custom skin', and select the skin file.

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UPDATE

At least for the moment, very few VLSub extensions are really working and are worth trying; and, among these, most are not actively developed, as you can see here: the best is VLSub, probably the best tool around at the present for downloading subtitles - to the point that recent versions of the player have integrated that addon by default (but in VLC 2.2.2 that is outdated and has stopped working: see below).

VLC 2.2. has the capability to search and install extensions within it's settings, under Tools-Plugins and extensions:

Only it doesn't always install the latest version which is needed for the extension to really work, in which case download the script from the developer (for VLSub, here) and install it as indicated in the first part of this answer.

If the addons that come with the player need to be replaced, look for them in /usr/lib/vlc/lua/extensions/.