It is a button, called 'Flip normals'. You see the names of the buttons, when you move the mouse cursor over it and stop. It is near the buttons 'Subdivide' and 'Extrude'.

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