Navigation Help

[streetfire]

I'm using the free Navigation Bar that is in the Resources section: http://www.studiopress.com/graphics/navigation-bars

I have successfully installed the code and everything works good. However, I'm trying to make the #nav div raise up 15 pixels on the page to overlay another graphic I have. The problem is when I do this the drop down menu's do not display correctly. Below is the only code I have added:

#nav {
background: url(images/nav-green.png);
clear:both;
color: #fff;
margin: 0 auto;
overflow: hidden;
text-transform: uppercase;
width: 960px;
position: relative;
top: -15px;
}

How can I resolve this so the drop down's will display?