You are either opening a non-NKS instrument in a free Kontakt Player, or haven't activated your NKS instrument in the Native Access. Some instruments we release can only be used with a Full version of Kontakt. Those instruments do not require a serial number and as long as it is the FULL version of Kontakt (NOT PLAYER), you will be able to use the instrument.For NKS instruments, please make sure you find your Order Confirmation email and click on the Download link which will take you to your serial and instructions how to authorise your new instrument in Native Access.