If you want to add packages to FreeBSD or OpenBSD (others?), you'll want to use the package collection called 'ports'.
# mkdir /usr/ports && cd /usr/ports
# portsnap fetch
# portsnap extract
This creates the directory heirarchy under /usr/ports and downloads the header files among other files for each package. When installing new packages, after the ports collection has been created on a given system, execute the following command to update the collection.
# portsnap fetch update
for more information,
go here.