"Installing" dbLink

Hi

 

When I execute the query:

 

SELECT *
FROM dblink('dbname=Bas','SELECT id FROM person') AS t(a int)
WHERE t.a > 9000 AND T.a < 9050;

 

I receive the error message

 

ERROR:  function dblink("unknown", "unknown") does not exist

 

How do I install or setup the function dbLink so that I can use this feature.

 

Regards

Richard