Put follwing code in your aspx page
<p>
<!-- AddThis Button BEGIN -->
<script type="text/javascript">
var addthis_pub = "test";
var addthis_brand = "test";
var addthis_header_color = "#ffffff";
var addthis_header_background = "#2689C7";
</script>
<a href="http://www.addthis.com/bookmark.php?v=20"
onmouseover="return addthis_open(this, '', '[URL]', '[TITLE]')"
onmouseout="addthis_close()"
onclick="return addthis_sendto()">
<img src="http://s7.addthis.com/static/btn/lg-share-en.gif"
width="125" height="16" alt="Bookmark and Share" style="border:0"/></a>
<script type="text/javascript" src="http://s7.addthis.com/js/200/addthis_widget.js"></script>
<!-- AddThis Button END -->
</p>
