You can't modify a wmb on its own, you can only change a wmp in WED, and build the wmb from that anew. (Hm, not sure, whether this is explained properly.)

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt