No no no. *sigh*
(NOTE: Assuming you have WINDOWS, here, no MAC or anything else...)
Along the top bar of any open window (notice the lowercase-"w"), such as Word or Notepad (or World Edit, or, in this case, Magos War3 Model Editor): You will see what is referred to as a "Menu Bar"; a toolbar located directly underneat the blue strip that serves as the "top border" to whatever window you have open. On this "Menu Bar", there are several selections; normally stuff like "File", "Edit", "View", "Help", etc, appears. However, in Magos case, you will see one labelled "Windows"
The way I wrote my instructions (a common-place, oft-used one) indicates that you should click once on the selection "Windows", then, on the drop-down list that unfolds from "Windows", select "MPQ Browser" (it just so happens to be at the very bottom, partitioned from everything else). Hence "Windows -> MPQ Browser"
Similarly, you open the "File" selection from the Menu Bar in the ensuing "MPQ Browser" window (small "w", again!), and select one of the four .mpq options (as detailed above) in order to access the different files in Warcraft 3 (as detailed above).
~~~
Please tell me I'm wrong in telling you "lrn2Windows"...