you may have set the executable path to the wrong file, or there may be a bug on my end! go to %APPDATA%\Godot\app_userdata\Level Juicer and delete the user_data file, then set it up again. This generally fixes the issue.
Also make sure you extracted the res folder and that there isn't another res folder inside the res folder, as situations like that can arise when unzipping folders.
You may also want to use the official level editor, which mostly succeeded this ancient one, aside from a few things you can only do through this editor.