ntop is available to install on FreeBSD via the ports system. If you haven't set it up yet, see this article for more information:

• How To Install and Manage Ports on FreeBSD 10.1

Once you've set it up initially, you can search for ntop by running:

sudo portsnap fetch update
cd /usr/ports
make quicksearch name=ntop path=/net

In the output, you'll find its path

Path:   /usr/ports/net/ntop

This lets us know where we need to run the installation commands from. Change to that directory, and install ntop from there using:

cd /usr/ports/net/ntop
sudo make config-recursive install distclean

This will configure, build, and install ntop and all of its dependencies.