To have snmpd start at boot time, you must edit ${SYSCONFDIR}/snmp/snmpd.conf
and uncomment the following in /etc/rc.local.

    #if [ -x /usr/local/sbin/snmpd ]; then
    #        echo -n ' snmpd';       /usr/local/sbin/snmpd
    #fi

This will start snmpd and use ${SYSCONFDIR}/snmp/snmpd.conf for the configuration. 
The snmpconf tool can be used to help edit your configuration files.  See 
snmpconf(1), snmpd(8), and snmpd.conf(5) for more options.