The portinstall program is just a front-end to the ports system, you don't need it to install ports.
For information on how to install port and packages see chapter 4 of the FreeBSD handbook
portinstall is part of the portupgrade package, first make sure if it is installed:
% pkg_info | grep portupgrade
If this doesn't return any results, you will have to install it, portupgrade lives in /usr/ports/ports-mgmt/portupgrade, see the link above how to fetch a ports tree and how to use it.