Hi
You seem to have 2 choices. You could either hard code it into the Javascript by finding where it says:
<div style="position:absolute; background:';
and adding in
font-family: Verdana, Arial, sans-serif;
for example, after the "absolute;" bit.
Or you could stick a div around the menu in the code like this:
<div id="mymenu">
<script language="JavaScript1.2" src="nav.js"></script>
</div> <!-- end mymenu -->
and then alter the font in the stylesheet like this:
#mymenu {font-family: Verdana, Arial, sans-serif;}
Obviously using whatever font-family you want! I'd tend to do the second one as it's less likely to go wrong.