To add this widget to your blog,
1.Log in to your dashboard --> Design -->Page Elements.
2.Click on 'Add a Gadget' on the sidebar.
3.Select "Follow by Email".
4.Now if you want you can add "FeedBurner URL" manually. but your feed address is automatically added to the "Feedburner URL" box by Blogger.
5. Click save. You are done.