Select Layout > Page Elements > Add a Gadget > HTML/JavaScript

Copy & paste below code:

<script style="text/javascript" src="http://h41r3z4n.googlepages.com
/recentcomments.js">
</script>
<script style="text/javascript">
var a_rc=5;
var m_rc=false;
var n_rc=false;
var o_rc=100;
</script>
<script src="http://yourblog.blogspot.com/feeds/comments/default?alt=json-in-script
&callback=showrecentcomments">
</script>