1. Tcpflow is not part of the default pfSense installation. Perhaps the easiest way to install tcpflow is to use the Linux/Unix repositories at https://pkgs.org: the 64-bit binaries for tcpflow v. 1.5.0 can be found at https://freebsd.pkgs.org/12/freebsd-ports-latest-amd64/tcpflow-1.5.0.txz.html. Install these into a suitable location on your pfSense system, such as /usr/sbin/tcpflow. Make sure the binaries are executable; you can do this with the following command:

 chmod 755 /usr/sbin/tcpflow

2. After you have installed tcpflow, the package is likely to have uninstalled dependencies that must be installed before tcpflow will run. The easiest way to install those dependencies is to consult a list of dependencies, and install them one by one using the pkg utility. For example, entering the pkg install pixman command into the command line will result in the pixman dependency being installed, if it hasn't already been installed.
3. Once tcpflow is installed, navigate to Diagnostics | Command Prompt.
4. In the Execute Shell Command text field, enter tcpflow, along with any parameters you wish to pass to the program.
5. Click on the Execute button.