Goto: Skin Manager > Edit Root Skin Template HTML > All Global HTML > global_board_header
Find
CODE
<div id="logostrip">
<a href='{ipb.script_url}'><!--ipb.logo.start--><img src='style_images/<#IMG_DIR#>/logo4.gif' style='vertical-align:top' alt='IPB' border='0' /><!--ipb.logo.end--></a>
</div>
Replace with
CODE
<div id="logostrip">
<div style="float:right;padding:5px">{AD_CODE}</div>
<a href='{ipb.script_url}'><!--ipb.logo.start--><img src='style_images/<#IMG_DIR#>/logo4.gif' style='vertical-align:top' alt='IPB' border='0' /><!--ipb.logo.end--></a>
</div>
Before saving the template, replace {AD_CODE} with the advertisment you wish to place there, an example could be
CODE
<a href="http://www.example.com"><img src="http://www.example.com/ad.jpg" alt="Advertisment" width="468" height="60" /></a>
(Remember to include the alt="" and ending / as this will kept your board XHTML compliant.)
Or if you wanted to use google adsense, just paste your code where {AD_CODE} is.
Please note that you should read the Google Adsense Terms of Service if you did want to incorporate Adsense in this way. You'll note that Google prefer you not to display their ads on non-content pages.