Add to your /etc/rc.conf
rblmilter_enable="YES"
Then run the milter with: $PREFIX/etc/rc.d/rbl-milter start

By default, it uses the relays found at relays.ordb.org but you can
change that by setting rblmilter_flags in /etc/rc.conf:
rblmilter_flags="-l -r -d relays.ordb.org"

Note that you will need to edit the proper sendmail.mc file in your
/etc/mail directory so that sendmail will pass mail to rbl-milter.
If you have not done this before, do the following:

  cd /etc/mail
  cp -p freebsd.mc `hostname`.mc

Add the INPUT_MAIL_FILTER line to the file:

  INPUT_MAIL_FILTER(`rbl-milter',`S=local:/var/run/rbl-milter')

and rebuild it

  make
  make install
  make restart

By default, the INPUT_MAIL_FILTER line will ignore the filter if
rbl-milter is not running.