Have someone create a campaign of decent length (like... somewhat short, but make it feel like it is complete and not a demo) whether it is a developer or someone from the fanbase... and include it in the game.
Let people open it up in the GM mode and look at how it was made.
Include some creative trigger work and basics but don't blow the potential creator away with complex scripts. Better yet attach notes in it for GM mode to look at for hints, tricks, advice, and tutorials on how to use GM mode.
You don't have to follow this exactly... but a example is invaluable.
That's actually a good one. Like perhaps "The King of Rattsburgh", where you play the roles of exterminators out to save the town of Rattsburgh from the wicket Rat King. One small town, one small dungeon, but enough content to show off the GM mode.