I recommend you to read HarryH's tutorial here: Mission editor tutorial
Executable file is WOFF UE_Mission_Editor.jar. The latest version (1.0.5) is here: WOFF UE_Mission_Editor.jar
Copy this file to your local Mission Editor folder downloaded here:WOFF UE Mission Editor
If you have properly installed Java on your machine, you can execute it simply by double click on WOFF UE_Mission_Editor.jar file.