If you want to show advertisement within the first post then there is very simple plugin postlite ad. It is little buggy which you can fix yourself: http://community.mybb.com/thread-125186-post-904769.html#pid904769.
Mind you, there does not seem to be any simpler way to do it. There is another but bit complicated way: http://community.mybb.com/thread-125188-post-904952.html#pid904952
A better idea to change this line 129 in the bottom:
$post['message'] = "<div style=\"float:right;\">{$ads}</div>{$post['message']}<div style=\"clear:both;\">";
to
$post['message'] = "<div style=\"float:left;margin: auto 1em 1em auto;\">{$ads}</div>{$post['message']}<br style=\"clear:both;\" />";
PHP 5.2/5.3 issue
This plugin is for PHP 5.2 but call by reference has changed in 5.3. So if your forum is using 5.3 then change the line 121:
function postad_start($post)
to
function postad_start(&$post)
That is just put one "&" before $post.
This issue is explained well in this thread.