If you opened the AddOns directory (C:/~Documents/Elder Scrolls Online/live~/AddOns) and extracted the add on in that folder, it will
probably be in the wrong spot. Many add ons actually put in an additional , unnecessary folder. You'll have to move the next folder down into the ~/AddOns folder. It will be a similar name, if not the same name (most have a version number in the name of the unnecessary folder, and no version number in the name of the folder you need to move).
If what you have is something like:
C:/~Documents/Elder Scrolls Online/live~/AddOns/
addonname x.x.x/
addonname/
addonname.lua
then change it so that it is
C:/~Documents/Elder Scrolls Online/live~/AddOns/
addonname/
addonname.lua
Thank you Cairenn for pointing this out.