You could try this :)
Open your skin_Arcade.php or skin_v3Arcade.php
Search for:
<td class="alt1" align="right" valign="top" style="width: 20%;padding-left:7px;padding-top:5px;padding-bottom:5px;">
<div class="tborder">
<table width='100%' border='0' cellspacing='1' cellpadding='4'>
<tr>
<td align="center" width="20%" nowrap="nowrap" class="tcat">{$ibforums->lang['infobox_mostplayed']}</td>
Above that put:
<fieldset class="fieldset" style="margin: 0px 0px 0px 0px;">
<legend>Advertisement</legend>
<div style="padding: 0px;">
<table cellpadding="0" cellspacing="0" border="0" align="center" width="100%">
<tr>
<td width="100%">
<table cellpadding="2" cellspacing="1" border="0" width="100%">
ADSENSE CODE</td>
</tr>
</table>
</td>
</tr>
</table>
</div>
</fieldset>
</td>
That would place the advert like in the attached image.