if you open the 3ds model files, the models are untextured. you need to import the texture that you've made and the generic, tyre, tread etc one after another. select project->materials. import all textures by selecting the paint part (like body_decal etc).
simple way. if you have the psd files, just export the models to recognizable format like .obj by selecting 3D-> Export 3D layers in Photoshop. place them to the folders that you want. and you get your models textured. open the obj file in keyshot and import the texture that you've made.
hope it helps you