You can use Warcraft 3 Viewer or any MPQ browser that can view models. Find the model you want and see what textures it uses (in wc3viewer, Ctrl + M). Those are the skins, use their paths in your imported skins to substitute them.
EDIT: ah, if your problem is with imported MDX, you should find the paths for the custom skins in the page you downloaded it. Note that not all textures included\listed are required, since some of them may already be in the MPQs.