Tom, if you've followed the procedures shown in the article linked
below, and still nothing happens, then I don't know what could be
causing the problem. I do know when you save icons to the toolbar and
close Word, they're saved to the normal.dot template, and that if you
have the full version of Acrobat, the Acrobat plugin to Word can
interfere with saving normal.dot. If it works from Insert > Object,
but not from the toolbar icon, I don't know what to tell you.

http://www.dessci.com/en/products/ee/ee_tips.htm#button_for_word

Also, the shortcut key to open Equation Editor from within Word
is Ctrl + Alt + Q. Maybe that'll work.