This should work:
HTML:
<div id="logo">
Some code here for the logo
</div>
<div id="ad-banner">
Ad banner goes here
</div>
<div style="clear:both;"> </div>
CSS:
#ad-banner
{
float: right;
margin-top: 60px;
}
Putting the HTML in the header-middle
div and the #ad-banner
style into the style.css
file. It should work.