I get the following error message:
ERROR: couldn't find oid for fxp1!
not sure what this means.
Most likely it means that your NIC has a different device name. For example, my NIC is rl0. (fxp1 would be an Intel NIC, rl0 is a Realtek NIC).
You can figure out which one you have by reading /var/log/dmesg.today (among other methods).
Once you figure out the correct device name, edit /usr/local/etc/rrdutil.conf and voila!
The easiest way to determine your NIC's device ID is simply:
ifconfig -a
That will display a list of all your network interfaces. It will work on any Unix like OS. On FreeBSD, you can just type "ifconfig" and it defaults to -a but I'm in the habit of -a since I work on quite a few unix variants.