ERROR: Unrecognized language specified ...

I'm creating a function (for use in a trigger) from the example in the
documentation and encountered an error. If I type the following at
the psql prompt (database and user are opf):

opf=> create function opf_user_fillin() returns opaque as '
opf'> begin
opf'> new.opf_user_id := nextval(opf_user_sequence);
opf'> return new;
opf'> end;
opf'> '
opf-> language 'plpgsql'
opf-> ;

I get the following error:

ERROR: Unrecognized language specified in a CREATE FUNCTION: 'plpgsql'.
Recognized languages are sql, C, internal and the created procedural languages.

Can I enable the plpgsql language on my system? If so, how? If not,
what are my options (ie, what do I have to do to get plpgsql support)?

I'm pretty new to PostgreSQL but no stranger to compiling systems from source
(or writing the source for that matter). I'm running a RedHat Linux 7.1 system
with the PostgreSQL that comes with the OS.

Thanks,
Kevin.