Please clarify - are you saying that you can't turn the key in the lock cylinder unless the key is button is pressed in, or are you saying that you CAN turn the key but it does nothing unless the button is pressed in?

If you can turn the key without pressing in the button then you probably need to adjust the length of something (but I'm not sure what). If you can't turn the key without pressing in the button then I think you have a worn or incorrect key or that the spring return on the button isn't applying any pressure. What happens if you hold the button out with one hand and try to the turn the key with the other?