Hi,
If it's a compressed file, just extract the zip/rar to X:\xxxxxx(where ever the allods online folder is)\Allods Online\data\Mods\Addons
(Make sure it hasn't extracted to an extra folder. Once you doubleclick on the addon folder you should see the actual add on folder structure, not a duplicte folder with the addon name).
If it's not compressed then just cut and paste the addon file to the same location.
Start the game, log in. Press escape and select addons. Just turn on the addon master switch and make sure each addon you want to use has a checked box.
Note: Some addons require other addons to work correctly. Please read the readme file that comes with each addon to check this.
Cheers!