That requires a dynamic menu. Publisher has no support for dynamic effects.

David Bartosik - [MSFT MVP]
www.publishermvps.com
www.davidbartosik.com

As David said there is no built in support for this in Publisher. However
you can get the effect I think you are looking for by inserting JavaSript.
You can google the many different JavaScript sites, or here is one example:

Modify and insert this code using the HTML code fragment where you would
like the box to appear.

<----->

<Script language="JavaScript">
<!-- Script courtesy of http://www.web-source.net - Your Guide to
Professional Web Site Design and Development
function goto(form) { var index=form.select.selectedIndex
if (form.select.options[index].value != "0") {
location=form.select.options[index].value;}}
//-->
</SCRIPT>
<FORM NAME="form1">
<SELECT NAME="select" ONCHANGE="goto(this.form)" SIZE="1">
<OPTION VALUE="">-------Choose a Selection-------
<OPTION VALUE="index.htm">Home
<OPTION VALUE="web_development.htm">Web Development
<OPTION VALUE="html_codes.htm">HTML Tips
<OPTION VALUE="html_codes_chart.htm">HTML Code Chart
<OPTION VALUE="javascript_codes.htm">JavaScript Codes
<OPTION VALUE="216_color_chart.htm">Color Code Chart</SELECT>
</FORM>

<----->

Here is a link to a terrific example of what you can do with Publisher 2000,
and it uses not only a drop down box, but also a third party program called
MenuMaker to build a dynamic menu. Look at some of their sample sites to see
some of the drop down menu effects that can be achieved with MenuMaker. I
have also used MenuMaker succesfully, but not as well as these folks:
http://www.karmawebdesign.com/index.html

Here is the link to MenuMaker: http://www.xara.com/products/menumaker/

DavidF