It is now simply a case of adding however many buttons to the navigation movie that you need. Create a shape (As I mentioned before it is best to make it fit within the mask) and make it into a button symbol. Add a layer to the navigation movie clip called buttons. Place as many instances of the button as you require on this layer. Right click the button and add a keyframe to the hit keyframe. Now go back and delete the content of the Up, Over & Down keyframes. This will create an invisible button that will activate when you hit the shape that you have put there.
These buttons can then be assigned any actions that you want.