If the Addon you are trying to install needs to be placed here - @ExileServer\addons\
This does not work in our system as we have the setup a little different.
First of all you need to create a folder named @Anything (Change the anything to something that explains the Addon)
For my example I am going to use @Zombies then inside this make a addons folder.
So now in the root of your server you should have @Zombies/addons/
Now you can upload the PBO files into the addons folder.
Once you have done all of the above you need to set the server to load this addon.
You can do this in the server manager which looks like the image below:
Now you have found the server manager you need to select advanced in the Access Level.This will then give you more options like the image below:
Now for the example I have been doing you would need to do something like the image below:
Note) if you have multiple server addons you will need to separate them with ;
With all the information above you should now be able to add server side Addons to your server.