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!