On the gerneral subject of addons, here’s a recent thread from the New Players forum that might be generally informative:
And here’s a very simple and friendly video introducing what some basic addons do:
Sadly, Blizzard changed the way rares are detected a couple of expansions ago, so NPCScan now requires you to mouse over the target.
You’d have to give us the name of the mount farming addon for us to guess at what it’s supposed to do. My furst guess is that you need to invoke it, with a command like /mount-farm-addon-name and it would then show you a list or map por something.