Steam->Editor
 

Appears like steam don't preview the direct launch of editor of game. When you launch it from game directory start the normal game not the editor. I have already signaled this to Steam support. I did information when they reply to me.

Steam support is without hope. But you can anyway uss the editor: changed the name of engine.exe (a letter different is sufficient) and rename the editor.exe in engine.exe. Launch it and the editor start. After have finished make the contrary.