Okay here is my step-by-step Guide for addons.
With addon manager.
Step 1: Download and install the twitch app for your OS. (you will need a twitch account to use the app)
https://www.twitch.tv/downloads
Step 2: Launch Twitch app and login.
Step 3: Click mods.
Step 4: Click Wow.
Step 5: Click Get more Addons
Step 6: Search or Browse for the addon you want and Click install then when you boot up the game it will there thats it its that simple.
Below is a video that runs through installing addons that I did. Forgive the quality I am not that good at speaking.