1) Download addon zip file from Riftui.com or rift.curse.com
2) Run Rift
3) Click on addons button on character selection screen
4) Click on open addon directory button on addons list screen - This will open a windows folder window
5) Copy the addon folder from the zip file (assuming that you know how to uncompress a zip file if not Google is your friend)
6) Click refresh once new addon is in the addon folder
7) Make sure addon is ticked and you are good to go.
You can play your characters now with the addon enabled.