wp-mailman integrates WordPress with the GNU Mailman mailing list manager.
wp-mailman heavily relies on correct file permissions. It executes programs on the web server to get information from Mailman, which is questionable, at least from a security point of view. However, there's no other way to get stuff out of the mailman control interface.
Best bet is to add the user that your web server is running under to the group mailman (or _mailman on some systems). Check your mailman installation for the correct value. wp-mailman does some checks to help you getting things straight and prints (hopefully) helpful error messages when needed.