[The Fifth Horseman]

Try to follow the tutorial first to get familiar with DOSBox: http://www.abandonia.com/vbullet/showthread.php?t=14406

Now, assuming you followed the tutorial, you have a C:\Oldgames directory that's mounted as your C drive in DOSbox on start.
Unpack Warcraft 2 from the archive into that directory.
Run war2.bat .
You're ready, enjoy the game.