if you have 2 definitions of the a tag, then your browser will most likely use the one that was declared last.
which one do you want to choose? whatever the answer to that is, then you probably have to include that script ( or style sheet ) last.
you may have to just open up mymenu.js in a text editor and find where they are using a normal a tag <a href=something.html>asf</a> , and add a new class in there <a class=nav href=someting.html>asf</a>
does that help?