Coloca assim:
<div class="widget">
<style="margin-top:15px;margin-bottom:15px;text-align:center;">
<script type="text/javascript"><!--
google_ad_client = "pub-4857784468840903";
/* 300x250, criado 25/07/11 */
google_ad_slot = "4464338952";
google_ad_width = 300;
google_ad_height = 250;
//-->
</script>
<script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</div><!--/widget-->