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://twitthis.com/twit?url=" + data:post.url' rel='external nofollow' target='_blank'>Twit this</a>
5.Click on "Save Templates" and now you are done.