Indeed, to add a model you need to adjust the ini of the unit you wish to add, as opposed to a central ini controlling models.
So, to add a model, put the model in Art\w3d and the texture in Art\Textures. If you are changing an existing model, simply place the new model with the correct name into the relevant folder.
If you are having trouble,
http://gendev.game-mod.net/ has some nifty beginners tuts, including adding new units.