because you are using one large image slice for the entire menu area. you need to break up the menu slices to the individual menu items.
you can use one large image slice for the menu navigation and add links, web page design does not work this way.
on the learning resources page:
tutorials-and-training/
you will find getting started kits for both CSS Designs and for CSS Menus, perhaps those will help point you in the right direction.