please dont curse, darkshadow >:'[
wc3 viewer allows you to load models for a mpq ( default ) or to load your model from an outside source.. and when you pick a model from the mpq, you can instantly apply the skin you made for a quick preview by pressing Ctrl+M to open up the active files, you change heroblademaster.blp ( or wahtever is your skin's name ) by the skin you made, it can have any name.
of course, before pressing ctrl+m, select your model in the treepath, if you dont know where it is, then seek, i wont awnser a such easy question unless for money