there is still no package munin-node for pfSense 2.3, but I did it this way (although it is not recommended, see: https://doc.pfsense.org/index.php/Installing_FreeBSD_Packages):

Enable FreeBSD repo - comment the lines saying "FreeBSD: { enabled: no }" in both files "/usr/local/etc/pkg/repos/FreeBSD.conf" and "/usr/local/etc/pkg/repos/pfSense.conf"

Install package - pkg update && pkg install munin-node

Change PID file path - in the munin-node config file "/usr/local/etc/munin/munin-node.conf" change the line "pid_file /var/run/munin/munin-node.pid" to "pid_file /var/run/munin-node.pid"

Enable autostart:

pfSense startup script must end with ".sh" - ln -s /usr/local/etc/rc.d/munin-node /usr/local/etc/rc.d/munin-node.sh

pfSense uses action "start", so regular starting must be allowed - echo 'munin_node_enable="YES"' >> /etc/rc.conf.local

Disable FreeBSD repo (rollback the first step)

Debug output of pfSense rc.d scripts starting is in the file "/tmp/bootup_messages"