Your locking plip should have 2 buttons on it:
1, Unlock the door with the key
2, Press and hold the large plip button (the light on the key should be flashing) wait for 20 seconds and press the small button on the plip once, the light should stop flashing
3, Release the large button, the light on the plip should stay on
4, press the large button once, the light goes out.
5, Enter the car, hold the key next to the ignition barrell, press the large button once
6, Switch the ignition on, wait for 10 seconds and switch off.
7, wait for 5 seconds, the plip should now operate.
If it doesn't you either have a faulty key or control unit (BSI) - go to a Peugeot dealer.
Mark, January 2006