…on the WordPress twenty eleven theme.
Just quoting this :
There must be two authors who have both published at least one post. While logged out of WordPress.com create another username account with a different email address. Then add the new username as an Author. Then log-in as the new author and publish one post and backdate it to the date you started the blog. Log-out and clear your browser cache and cookies.
Log in under the original username and you will have an author byline and an author profile below each post. The data for the author profile is from this page > Users > My Profile « About me »