It's super easy to install the addons. Once you have downloaded the .maddon file. OPEN IT! Minecraft will reconise the file and begin to load.
Once you open Minecraft you will see "Import Started" At the top of your screen. Take a look at the screenshot. Once it has imported you will get a message to say Sucess!
Go into your world edit (or create a new world) Under Game settings. Make sure EXPEREMENTAL GAMEPLAY is turned on.
Activate the Addon in BOTH Resource packs and Behaviour Packs by clicking the activate button.
Your Game Must be in creative mode in order to find the new addon in the creative menu.
World spawning is not currently supported (but coming soon!)
if you're having trouble finding new Entities
The new addon items can be found with the EGGS in the nature tab under creative mode.