At the moment I've got this trigger:

CREATE OR REPLACE TRIGGER v_med_trig
INSTEAD OF INSERT OR UPDATE ON V_MEDEWERKER
BEGIN
INSERT INTO MEDEWERKER
(medewerkernummer, functie, voornaammedewerker,
achternaammedewerker)
VALUES
(:new.medewerkernummer, :new.functie, :new.voornaammedewerker,
:new.achternaammedewerker);
INSERT INTO MEDEWERKERTELNUMMER
(medewerker, telefoonnummermedewerker)
VALUES
(:new.medewerkernummer, :new.telefoonnummermedewerker);
END v_med_trig;

Now it's my intention to build in some kind of sequence. Does anyone know how to do this. When I try myself oracle says it can't update this view.

I'm unable to understand what you are trying and what goes wrong exactly. Can you post the modified trigger code that is failing, and the error message?

Oke, I'll try to explain my problem a little better. The trigger you see above is a trigger that makes it possible to update a view. If you look at the columnnames you see the name medewerkernummer.
At the moment I have to give a number to every new record I make (I have to check if a number is used everytime). So, to solve this I made a new trigger:

CREATE OR REPLACE TRIGGER NIEUWEMEDEWERKER
BEFORE INSERT OR UPDATE ON MEDEWERKER
FOR EACH ROW
BEGIN
SELECT sID.NEXTVAL INTO :NEW.MEDEWERKERNUMMER FROM DUAL;
END NIEUWEMEDEWERKER;
/

But when I use this trigger the view doesn't seem to be updateble anymore. I've got the feeling that this sequence has to be build in the trigger I posted first in this thread but I don't have clue how to do this.

I'm not aware of such a restriction, but that could be the problem. Anyway, you can do the assignment in the original trigger like this:

CREATE OR REPLACE TRIGGER v_med_trig
INSTEAD OF INSERT OR UPDATE ON V_MEDEWERKER
BEGIN
INSERT INTO MEDEWERKER
(medewerkernummer, functie, voornaammedewerker,
achternaammedewerker)
VALUES
(sid.NEXTVAL, :new.functie, :new.voornaammedewerker,
:new.achternaammedewerker);
INSERT INTO MEDEWERKERTELNUMMER
(medewerker, telefoonnummermedewerker)
VALUES
(:new.medewerkernummer, :new.telefoonnummermedewerker);
END v_med_trig;