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,