The editor is in there, but the open beta isn't moddable and the editor won't let you save. You can layout textures and objects and figure out how most everything works though. You can also open the levels that you play in the game if you want to see fully build levels. Just a warning, you will find spoilers.
In the final game, the editor will probably unlock when you beat all of the adventures. For now though, just bring down the console (~ key), type in developer 1 and then hit enter, and then click on the editor button in the top right hand corner (if you are on the main startup screen).
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, & Din's Legacy