If you want to add a feed that shows your Recent Posts in the sidebar, you can use the BuzzBoost feature from FeedBurner (www.feedburner.com). It is very simple.
First, burn your feed at FeedBurner. Then go to your feed's homepage on FeedBurner and click the Publicize tab. From that section select BuzzBoost. (See figure below)
That's it. View your blog and you will see the recent posts appearing on your sidebar.