0.- In the meantime freepbx is not in the oficial freebs ddistribution download the port from here
http://www.freepbx.org/attachment/ticket/761/freepbx-2.1.tar
1.- Choose what apache, mysql and php version you want to use. For example, you could put this in your make.conf:
DEFAULT_PHP_VER=5
DEFAULT_MYSQL_VER=50
APACHE_PORT=www/apache20
2.- cd /usr/ports/misc/freepbx
3.- If you have php previously install make sure you have pear in your php.ini include paths.
3.- make install
4.- drink a coffee
5.- Read the post instalation notes:
1) enable .php files in your apache config
2) adding index.php as default index files
3) add pear and /.../admin to your php.ini includes
4) Make sure asterisk and mysql are running fine
5) Make sure asterisk accepts connections (manager.conf)
7.- Complete this document!