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

if [ -x ${PREFIX}/sbin/snmpd ]; then
	echo -n ' snmpd';	${PREFIX}/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.