[LifesGreatestGift]

Open SHOWTHREAD

find

<div id="qr_defaultcontainer" class="blockfoot floatcontainer qrcontainer<vb:if condition="$show['qr_require_click']"> qr_require_click</vb:if>">

Add before:

<style type="text/css">
#qr_defaultcontainer {
    float: left;
    width: 73%;
    clear: none;
}
#qr_ad {
    width: 25%;
    float: left;
    height: 300px;
    background-color: #888;
    color: #FFF;
    margin: 0 1%;
}
</style>
<div id="qr_ad">
AD CODE
</div>

Work with it til it fits what you need. If you have a fixed style, then use pixels instead of a percentage for width.