Install snmp:
$ pkg install system/management/snmp/net-snmp

Enable snmp:
$ svcadm enable svc:/application/management/net-snmp:default

Make sure snmp works:
$ snmpget -v2c -c public localhost sysUpTime.0

Do some networking traffic:
$ while : ; do rm -f /home/vmarek/tmp/gs ; cp /usr/bin/gs /home/vmarek/tmp; done &

Test mrtg:
$ rm -rf /var/tmp/mrtg && mkdir -p /var/tmp/mrtg
$ cfgmaker --global "workdir: /var/tmp/mrtg" public@localhost > /var/tmp/mrtg.cfg
$ set -x; for i in 1 2 3 4 5 6 ; do env LANG=C /usr/bin/mrtg /var/tmp/mrtg.cfg; sleep 600; done

After 30 minutes display the generated html in /var/tmp/mrtg. You should see
some data.