Ok, I'm not even talking about the expansion map editor, I'm talking about any map editor, I got HOMM2 Gold Edition and finally got round to playing it but I have no map editor.
In my HOMM2 Gold files I have "EDITOR2" file which is 823KB, I open it, it requests permission to open, I give that and then it says that fullscreen mode is not supported (it is not set to open fullscreen). There is another file in this folder called "dosboxhomm2_editor.conf" (7.17KB) which when clicked on says "Windows wants to know what you want to open this program with it can either search online or you select from a list" but basically that won't open either.
Do I even have a map editor? Any clue at all? I'll put a bet on it being something really dumb and simple like last time.

Just mentioning this, since this was the reason one of my maps wasn't appearing and it was driving me crazy: Did you make sure it has at most 1 player marked as Human-only. If it has more than 1 human-only player, it will only appear in multiplayer games.

Otherwise, are you sure you're putting the maps in the correct folder? See H2IWclassic's post above.

Hi all Had the same problem but just solved it ;)
You need to download the Optional Windows version of the game NOT the DOS version
The Win version has the Map editor and you open through the start menu and go to GOG games - HOMM2 Win version - Editor
Good Luck Byron