foreign keys

Greetings,

we have been trying to include into a couple of tables a definition to
use foreign keys from other tables. We tried the way just below and as
result we got a statement that

NOTICE:  CREATE TABLE/FOREIGN KEY clause ignored; not yet implemented
NOTICE:  CREATE TABLE/FOREIGN KEY clause ignored; not yet implemented

create table Bestellung (
 Bearbeitungsnummer        int4 not null primary key,
 Bestellnummer            int4 references Spiele_pkey,
 Kundennummer            int4 references Kunde_pkey,
 Lieferstrasse            text,
 Lieferplz            text,
 Lieferstadt            text,
 Bestelldatum            datetime not null,
 Preis                money not null,
 Datenmedium            text not null,
 Lieferdatum            datetime not null );

And the question now arising to the two of us doing this is: how do we get
something like foreign keys included into our tables at all?

Any helping pointers or solutions will be accepted ;-)

Thanks for helping,