If you install from RPMs and install the postgresql-tcl RPM too, then the bboard search is automatically enabled for you.
If you need to install 'by hand', then you have to change that param Don mentioned, and do
su - postgres -c "createlang pltcl yourdbname"
psql -q -U yourdbuser yourdbname </web/yourservername/www/doc/sql/rank-for-search.sql
too. Adjust the values of yourdbname, yourdbuser, and yourservername to match your own installation of course!
