To open up levels in the editor go to the menus and press CTRL-O to open a world.
There are also other tools like the model viewer:
A fan-made game combining Lego and Wolfenstein 3-D using the C4 game engine.
How to open up Lego Wolf3D levels in the world editor.
Posted by legowolf3d on - Basic Mapping/Technical
To open up levels in the editor go to the menus and press CTRL-O to open a world.
There are also other tools like the model viewer: