To make use of postgrey, edit your configuration files:

/etc/postfix/main.cf:

  smtpd_recipient_restrictions = ...
    check_policy_service unix:postgrey/socket, ...


Or if you like to use inet sockets (modify the IP if needed):

/etc/sysconfig/postgrey:

  options="--inet=127.0.0.1:10023"

/etc/postfix/main.cf:

  smtpd_recipient_restrictions = ...
    check_policy_service inet:127.0.0.1:10023, ...

And don't forget to activate the service on startup!