OK, real simple!
After you enter your content info into the body of your post, look down the page a bit for the SOURCE and SOURCE URL BOX.
Just enter the name of the source and the URL.
Oh, and if you know the actual author of the source, also enter their name in the VIA box and VIA URL as well. Its a great way to show on the article that you got info from this source. It's very pro.
Give it a try!
Here's the screenshots