No matter where you keep the game, proper addons location is in fact C:\Users\< username >\Documents\Elder Scrolls Online\live\AddOns
This is separete location from game folders

BUT -if you migrated to EU server you need to copy your Addons folder to .../Liveeu/Addons
It was similiar surprise for me when after migrating servers all my addons sudenly dissapear

No, cos they need to be in document folder, not in game folder

There's multiple ways to have your addons on your external drive.

One is to move the location of your my documents folder (Requires registry edits or third party tools)
Easiest is probably to just junction it - make the folder into a virtual link to your external drive:
http://en.wikipedia.org/wiki/NTFS_junction_point

Same here my addons aren't showing up as per above comments do they have to be in live and live Eu or just one or the other. Also do we copy every single file of the downloaded files into the folder or just a select few. I am trying to get Pawkettes loot and the Foundry Combat adds working

Is that seperate top the Addons button on the main menu then?

Ok cheers i will have a look - but mine aren't even showing up in the Addons menu not looked at the settings one so i will see what happens when i get home from office - wife has banned me from playing it in the office !! Boooooo

So my girlfriend discovered that if you follow the below instructions, which were taken directly from ESOUI.com, everything works fine. My problem was that i wasn't extracting the files to my Desktop then dragging/dropping them into the AddOns folder. I just extracted them straight to the Addons folder. I have no idea why the extra step is needed but it worked! Hopefully this helps anyone else having troubles.

To manually install your AddOns:

1. Download your chosen addon

2. Extract the files to a folder on your desktop.

3. Open your ESO folder.

If using a PC, the typical default install path is:

C:\Users\< username >\Documents\Elder Scrolls Online\live\

If using a Macintosh, the typical default install path is:

~/Documents/Elder Scrolls Online/live/

4. In the 'live' folder, create a new folder called AddOns (if it doesn’t already exist)

5. Open the AddOns folder

6. Copy the files from the folder on your desktop into the AddOns folder

7. Your final path should look like:

PC:

C:\Users\\Documents\Elder Scrolls Online\live\AddOns\< AddOn A FolderName >\< files >
C:\Users\\Documents\Elder Scrolls Online\live\AddOns\< AddOn B FolderName >\< files >
C:\Users\\Documents\Elder Scrolls Online\live\AddOns\< AddOn C FolderName >\< files >

Mac:

~/Documents/Elder Scrolls Online/live/AddOns/< AddOn A FolderName >/< files >
~/Documents/Elder Scrolls Online/live/AddOns/< AddOn B FolderName >/< files >
~/Documents/Elder Scrolls Online/live/AddOns/< AddOn C FolderName >/< files >

(where < AddOn A Folder > < .B. > < .C. > are the names of the actual folders that the addons create)

9. Log into game. At the character select screen, there should be an "AddOns" button in the lower left corner. Click on that. If any of the mods in the list that pops up say "out of date" or some such, choose the "Load Outdated AddOns" option.

10. Log the rest of the way into game, you should be up and running now.

Well my 2 cents..
The game is still a tad unstable so why 'possibly' cause further
problems at this time? No addons for me for awhile anyway..
cheers....

Ok, I just got it working. The reason copying them from the desktop seemed to work is that you probably copied the subfolder into AddOns. Simply extracting the zip file into AddOns creates an extra directory.

For example, extracting a FooMod_v1.2.3.zip into AddOns gives:

AddOns/FooMod_v1.2.3/FooMod/(plugin guts)

while ESO expects

AddOns/FooMod/(plugin guts)

Move the extracted subdir directly into AddOns, restart the client, and it should work.