Install the port:

cd /usr/ports/net/mrtg

make install

make clean

If you dont already have an mrtg.cfg to restore, try this:

cfgmaker localhost > /usr/local/etc/mrtg/mrtg.cfg

edit the first few lines of /usr/local/etc/mrtg/mrtg.cfg

The first couple of times you run mrtg, you will get some error messages as it generates new data files:

mrtg /usr/local/etc/mrtg/mrtg.cfg

Add the above command to /etc/crontab similar to this: