# mkdir /usr/local/etc/openvpn
# cp your_openvpn_config_file.conf /usr/local/etc/openvpn/openvpn.conf
# sysrc openvpn_enable="YES"
# sysrc openvpn_if="tun"
# sysrc openvpn_configfile="/usr/local/etc/openvpn/openvpn.conf"
# sysrc openvpn_dir="/usr/local/etc/openvpn"
Then reboot or:
# service openvpn start