After updating the freeBSD ports collection follow these few simple steps to install asterisk
1.) cd /usr/ports/net/asteriskXX(XX may be 10,14,16 depending upon the version)
2.) make
3.) make install clean
and done..
Important: If you want to install and setup PRI ( dahdi,libpri etc) on freeBSD systems, its not going to work because currently only asterisk supports freeBSD well though DAHDI is also being developed to support it. As far as Digital telephony cards are concerned, most of them do not support freeBSD (or BSD also).
Ashish Gautam 
Shortly coming up with simple PRI installation and configuration steps for asterisk with dahdi