It is greyed out, if you didn't choose the vertices mode.
And it is perhaps hidden, in case that you use MED in window mode.
The weld button is between the merge and the mirror button.
Move slowly over the buttons to get the name of it shown.

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