DBLink issue

I would like to use dblink (postgres 7.3.5) to synchronize the data between two databases.
I installed dblink and run the command "/usr/local/pgsql/bin/psql -U postgres template1 < dblink.sql"
and everything seemed to work fine.
When I tried to use the command "select *from dblink(hostaddr=127.0.0.1 dbname=DB1 user=postgres', select username from
users)as t1(username varchar);", I get the following message: 
  ERROR:  Function dblink("unknown", "unknown") does not exist
          Unable to identify a function that satisfies the given argument types
          You may need to add explicit typecasts

Could anyone tell me what might be wrong?

Thanks