in mapeditor, press ctrl r any where in a blank space and change map type, i was told not to change the map type though as it f***s up the game (optec told me ages ago)

I have created my sigle player mission by creating multy player map with with positioning player and few ai hard. Then I started that intsant action, paused it and then go to edit mode. Now you can do whatever you like to do. When finished, exit from edit mode, so you you'll be back to playing and save that instant game. Now you have something like single player mission.

The main reason why you have to create it with instant game is because if you put in map setting that this map is SP, the AI and all rules, objectives are controled by external scripts which we can't edit or create, because they are hard coded.

For example go to map exchange section of forum, and take a look at my map.