There is no built in support for dynamic menus in Publisher. You can use the
Insert HTML Code Fragment tool to insert code, and build your own. Google
for dhtml menus, javascript menus, flyout or dropdown menus, etc, and you
will find lots of free script and code that you can insert into your
Publisher document. Some work...some don't.

Don't forget that Publisher is intended for fairly simple static websites,
and if you continue to seek dynamic features, then at some point it might be
wise to consider FrontPage or some other tool.

DavidF