xicer please read some tutors, that what they r for. well ill just explain. if u have a skin for a normal wc3 model, find the path (that thing u have to write in) in object editor at art - model. if its for a custom model, people mostly give it the path like draco mentioned.
the model editor, u probably mean 'magos model editor' right? well its mostly for editing particles, bones, sounds etc. for models from skretch u will need oinkerwinkle tools or 3d's max or something like that