Go to the main page of Plex Web
on the left side of the screen
take a look at the menu item ‘Channels’ (depending on the number of libraries you’ve created, you might need to scroll all the way down on this left side of the window)
Gerbowers is right. There is no longer an install button in the Channels section as there used to be in previous versions. I am on version 1.10.1.4602 of the Plex Media Player and the Plex Media Server. I am unable to add the Sub-zero agent unless I install it manually, but then I assume I will have to keep it up to date manually too.