Assuming top left – make a new file header.php in your child theme with this code in it:
<!DOCTYPE html>
<!--[if IEMobile 7 ]><html id="blaskan" class="no-js iem7" <?php language_attributes(); ?>><![endif]-->
<!--[if lt IE 7 ]><html id="blaskan" class="no-js ie6" <?php language_attributes(); ?>><![endif]-->
<!--[if IE 7 ]><html id="blaskan" class="no-js ie7" <?php language_attributes(); ?>><![endif]-->
<!--[if IE 8 ]><html id="blaskan" class="no-js ie8" <?php language_attributes(); ?>><![endif]-->
<!--[if IE 9 ]><html id="blaskan" class="no-js ie9" <?php language_attributes(); ?>><![endif]-->
<!--[if (gt IE 9)|(gt IEMobile 7)|!(IEMobile)|!(IE)]><!--><html id="blaskan" class="no-js" <?php language_attributes(); ?>><!--<![endif]-->
<head>
<meta charset="<?php bloginfo( 'charset' ); ?>">
<title><?php blaskan_head_title(); ?></title>
<?php wp_head(); ?>
<?php if ( is_singular() ) wp_enqueue_script( 'comment-reply' ); ?>
</head>
<body <?php body_class(); ?>>
<?php echo blaskan_top(); ?>
<div id="site">
<div id="wrapper">
<div id="ad">put my ad code here</div>
<header id="header" role="banner">
<?php echo blaskan_header_structure(); ?>
</header>
<!-- / #header -->
Then in your new child theme style.css file, add this:
#ad {
width: 728px;
height: 90px;
border: 2px solid red;
}
Obviously, change the border or size as you wish.