From OpenNMS

Download the port for OpenNMS from:
http://www.geeklan.co.uk/files/opennms/opennms-162-freebsd-port.tgz

extract the file to ports/net-mgmt/
tar -zxvf opennms-162-freebsd-port.tgz -C /usr/ports/net-mgmt/

install opennms
cd /usr/ports/net-mgmt/opennms
make install

All the dependencies will be downloaded & installed, you may need to fetch some files manually to build a jdk, you will be given exact instructions if that is the case.