Where do I need to install/extract the addon zip files to?
If you are not able to locate the correct folder where to extract the addons to you will find the information here: How to install an addon
Why can't I see any addons inside the game's AddOn manager?
1. You have moved your documents folder to another drive e.g.
Search for UserSettings.txt or AddOnSettings.txt file on your harddrives/sdds and check which folder it is in (with the most up2date date and time!).
Below this folder, in the subfolder AddOns, your addons need to be located so the game will find them as well.
2. Microsoft OneDrive
If you are using Microsoft OneDrive and your "documents" folder was moved to the OneDrive be sure to remove the Elder Scrolls Online folder from it again and put it into the normal documents folder (below windows user folder) again. OneDrive is not supported within ESO!
Additional information can be found in multiple ESOUI and also the official ESO forum threads (e.g. here)
3. Microsoft Antivirus (Windows Defender) is blocking the documents folder against changes
After an update to the Microsoft Windows 10 Antivirus tool Windows Defender the tool is blocking access to the folder users/dcouments somehow. You have no write access (not always at least) and thus the file AddonSettings.txt or the SavedVariables folder in the live folder cannot be updated properly.