Please see the text from the KLM website:
My flight was scheduled for April 23, 2010.
http://www.klm.com/travel/nl_en/cust...ions/index.htm
Refund of ticket costs
If you were unable to travel and would like to request a refund of your ticket costs, please use this form. KLM will provide a full refund for all unused and/or partly used tickets with a flight date from 15 to 23 April inclusive. It is important that you first contact KLM Reservations (020-4 747 747) to cancel your flights. This will ensure that the seats become available for other passengers.