You can place your advertisement via advance theme. Place below code into your qa-theme.php. If it is SNOW theme than somewhere around after line #116. However place doesn't matter
function q_view($q_view)
{
qa_html_theme_base::q_view($q_view);
$this->output('YOUR ADVERTISE HERE'); // here you can use html tags or simpel string
}
This will display yoru advertise after question box and before answer starts