Inside your textbox add the following merge tags:
Each of these elements pulls in a different part of your RSS feed or adds different functionality. There are a bunch of merge tags that you can use documented in MailChimps Knowledgebase.
The first thing to change is the FEEDURL. Replace this with the URL of the RSS feed you want to display in your campaign. As you go down the tags you can see we are specifying the count of posts with the [$count=3]. Change this to however many posts you would like to display. Obviously the more posts you display the longer your email.
One thing that we specifically wanted was to have our title be a link to the article being displayed. To do this we needed to modify the source code of our campaign by adding in the line:
This embeds our post URL as a link while displaying the title of the post as the link text. Remove the quotes at the beginning and end if you are going to use this.
The last thing to consider is formatting. Simply entering in these merge tags will display as regular text. Use the MailChimp built-in editor to pretty up your design.