First, go to your Deadlock installation folder by right-clicking the game in your Steam library and selecting Manage > Browse local files.

Open the gameinfo.gi file located in game/citadel with any text editor. Find the FileSystem section and modify it like this:

gameinfo.gi
    FileSystem
    {
    	//
    	// The code that loads this file automatically does a few things here:
    	//
    	// 1. For each "Game" search path, it adds a "GameBin" path, in <dir>\bin
    	// 2. For each "Game" search path, it adds another "Game" path in front of it with _<language> at the end.
    	//    For example: c:\hl2\cstrike on a french machine would get a c:\hl2\cstrike_french path added to it.
    	// 3. If no "Mod" key, for the first "Game" search path, it adds a search path called "MOD".
    	// 4. If no "Write" key, for the first "Game" search path, it adds a search path called "DEFAULT_WRITE_PATH".
    	//
 
    	//
    	// Search paths are relative to the exe directory\..\
    	//
    	SearchPaths
    	{
    		Game                citadel/addons
    		Mod                 citadel
    		Write               citadel
    		Game                citadel
    		Write               core
    		Mod                 core
    		Game                core
    		AddonRoot           citadel_addons
    		OfficialAddonRoot   citadel_community_addons
    	}
    }
    AddonConfig
    {
    	"UseOfficialAddons" "1"
    }

Note

This file occasionally gets updated, so you’ll have to edit it again.

After saving the file, you can place mods in Deadlock/game/citadel/addons.

Mods have to be named pakXX_dir.vpk where XX is a number between 01 and 99. Mods with lower numbers have higher priority. For example, if you have two model replacement mods for the same hero, the one with the lower number will be used.