This is how to install apps in PLEX. First goto the following url: http://127.0.0.1:32400/web/index.html#!/channels where you should see the following:
Click on Directory as indicated above and you should see the following:
We will use the BBC iPlayer for this tutorial, so click on the BBC iPlayer icon above and you will see something like this:
Click on the Install button above after a short wait you will be presented with the following:
From this screen you can Update or Uninstall the plugin. Close the BBC iPlayer window by clicking on the x at top right of the BBC iPlayer window.
The orange tick indicates that the plugin is installed, now click on the home icon top left black strip (the house).
Thats it you have installed BBC iPlayer app.