Search for UserSettings.txt on all harddrives and see which folders you get.
In there should be the AddOns folder and in there the addons.
Or read this wiki thread:
Find the AddOns folder inside your ESO folder (if it doesn't exist, start up the game once, and it will create it for you):
To find the folder "AddOns" read this wiki entry:
https://wiki.esoui.com/UserFolder
It is a subfolder of the UserFolder.
Try the Explorer link mentioned in there to open your documents folder where the elder scrolls online folder should be in.
Are you using Minion to install addons? Does it find any folder? If not reset the Minion.xml file (search and delete it) and start with a new Minion try.
If you are installing the addons manually:
Be sure that the extracted ZIP files only extract the folder INSIDE the zip archive, and won't create a folder named like ther zip archive + then put the contents of the zip archive in it.
e.g. zip archive name is "FCOItemSaver_v1_4_51.zip". In there is a folder FCOItemSaver.
If you extract the zip archive now to the AddOns folder (e.g. c:\users\philliy\documents\Elder Scrolls Online\live\AddOns) it should only contain the folder FCOItemSaver.
But it should not look like this: FCOItemSaver_v1_4_51\FCOItemSaver
Code:
Correct:
c:\users\philliy\documents\Elder Scrolls Online\live\AddOns\FCOItemSaver
Wrong:
c:\users\philliy\documents\Elder Scrolls Online\live\AddOns\FCOItemSaver_v1_4_51\FCOItemSaver