If you are talking about short-haul intra-European KLM or KL cityhopper service, you cannot select seats more than a day or so ahead (and possibly not until you get to the airport). Neither can anyone else.

If you are talking about inter-continental KL service, you can select seats on nwa.com, but only using the MyInfo seat selection system, not the ViewReservations seat selection system.

see also the following threads, among many others on the subject:
http://flyertalk.com/forum/showthread.php?t=403636
http://flyertalk.com/forum/showthread.php?t=390969
http://flyertalk.com/forum/showthread.php?t=373100
http://flyertalk.com/forum/showthread.php?t=385121
http://flyertalk.com/forum/showthread.php?t=383971
http://flyertalk.com/forum/showthread.php?t=373491