1.Log in to your dashboard--> layout- -> Edit HTML
2.Click on "Expand Widget Templates"
3.Scroll down to where you see this:
<p><data:post.body/></p>
4.Copy below code and paste it immediately after the line <p><data:post.body/></p> .
<a expr:href='"http://www.stumbleupon.com/submit?&url=" + data:post.url + "&title=" + data:post.title' rel='nofollow' target='_blank'>Stumble it !!!</a>
5.Click on "Save Templates" and now you are done.