lo_export

Can someone give me an example for lo_export? I am trying to export a
file from a database using the pgAdmin 1.6 (Windows). I am trying the
following query:

SELECT lo_export(attachments.content, '/test.txt') from attachments
where id = 1;

...but all I get is an error with the function name or argument
types. The help file shows that there are 3 arguments (PGconn *conn,
Oid lobjId, const char *filename), but I cannot find anything that
defines what to use for the PGconn.