AFAIK, you cannot add buttons by non-2007 add-ins to the QAT. It works
fine for add-ins that customize the Ribbon using the 2007 meachanism
(RibbonX). I don't know if this is a bug or not.
EndNote 7 works fine for me in Word 2007. It is on the add-ins tab only
though. Which EndNote version do you have?