in the definition of the menu item <li> to achieve this effect. It depends on a few other variables being set (or changed to reflect your situation), but the result should be the class='selected' being applied to all of the items in the menu that are part of the current path.
In ezwebin (and should also be in ezflow), there is two css classes you can use. 'current' is on menu element for the node you are currently on, and 'selected' is the on menu elements the is in the path you are in.
So you can have look in the code used there to see how it's done: http://svn.ez.no/svn/extensions/ezwebin/trunk/packages/ezwebin_extension/ezextension/ezwebin/design/ezwebin/templates/menu/flat_top.tpl