Use a widget area and place your ad in the widget (with one of the many adsense plugins, or use the text -html plugin en copy the code directly). E.g. my header area config looks like this:
%bar1 %logo %bar1 <?php bfa_widget_area('name=topwidgetarea&cells=2&width_ 1=730&width_2=230&align=8&align_2=6&before_widget= <div id="%1" class="header-widget %1">&after_widget=</div>'); ?> %bar2 %cats %bar2
This gives me two "topwigets", one with a width of 730 px and one with 230px, enough space for two ads (big and small); I only use the big one and leave the small one empty. See
www.diskidee.nl
If you want the ad above the logo place the wiget area before %logo ...