Go to your current drive, example C:\ then find program files (x86). There locate the directory "GOG Galaxy". Once you open that directory your going to look for the directory labeled "Games". Open that and look for "HoMM4 Complete". Open that directory and your should see the editor there labeled "campaign_editor". Click that and you'll be in the map editor for the game. I pinned it to my start (If you can) since I use it a lot because even though this game is old, its still money.