Okay, well then I can't tell you exactly what is wrong.
I can tell you that you can't directly z-index the content of an iframe unless you call a specific stylesheet inside the head of the iframe. For security reasons, iframes cannot be manipulated with css that is called from outside the frame.
Also, in order for z-index to work, the element that you add the z-index to must also have a position defined as something other than static. You must declare "position: relative", or absolute or fixed.
If the skin is indeed using superfish, you don't just want to z-index the menu, you want to z-index the container of the menu. So if your menu is in your header, you need to do ".headerclass { position: relative; z-index: #; }" where # is higher than the z-index of the element that contains the iframe.
Hope this helps,
-Isaac