If you're talking about custom models, this will help you:
-Open the World Editor
-Open the Import Manager (F12)
-Import the Model (and Skin if the model need it)
-When you import them, will apper "war3mapImported/ModelName.mdx" and "war3mapImported/SkinName.blp", just remove the "war3mapImported/"
-Save the map
-Close the World Editor
-Open it again
-Open you map
--DONE--
If you're talking about custom skins, follow these steps:
-Open the World Editor
-Open the Import Manager (F12)
-Import the Skin
-When you import them, will apper "war3mapImported/SkinName.blp", change the path to the model that you wish to replace the skin. Example: If you're using the
Crystallya skin, then change the path to "units\Human\Sorceress\BloodElfSorceress.blp"
-Save the map
-Close the World Editor
-Open it again
-Open you map
--DONE--