'pure-ftpd' is a sample PAM configuration file, to be copied in your '/etc/pam.d/' directory, or if you don't have one, at the end of your /etc/pam.conf file.
'ftpusers' is a list of users that aren't allowed to log in. Copy that file to '/etc/ftpusers'.
To enable PAM, don't forget to configure with --with-pam.