WebsiteBaker has a pretty good help area online:
http://www.websitebaker2.org/en/help/designer-guide/multiple-menus.php#anker3On this page you can see all parameters for the "old" show_menu() function.
There it shows the 10th parameter will be the class for the current menu.
In your call this is forced to "" (nothing).
By simply removing the last 2 parameters from your functioncall it should show " class="menu_current"" in the current active menupage.