Chrisco,
It should ask you for a password before giving you the forbidden message. I am guessing that you have the httpd.conf file changed to require a user
grep "require user" /etc/httpd/conf/httpd.conf
You should see one or more lines that tell you admin is the required user. You may want to look into that file and make sure this is actually under the phpmyadmin seciton.
Did you restart the web service?
/etc/init.d/httpd stop
/etc/init.d/httpd start