I'm a big fan of map creation, as well as of playing maps other people have created, but I don't see an option for the map editor on the main menu or on the Conflux menu. Does a map editor exist?
It sure does! However, unlike in past Heroes games, it's not accessible via the game itself, but rather as a separate program.
By default, this application is buried deep within the bowels of your steam folder.
If you've used Steam's default install settings, like I have, you should find it here:
It's the innocuously named "editor" application.