Postby Cedur » Thu Feb 08, 2018 11:46 am
You right-click for a level's properties on the world map, where you can set which paths will open upon which exit type. Then you can e.g. set one direction to "SMW Key" and this direction can serve as a path to a secret level.
Also since I assume you're using the legacy editor (1.3.0.1), make sure to get 2.0 and PGE. It's a thousand times smoother to work with.