Noordwijkerhout is 25 miles from Amsterdam and it's a pain to get to!
From Schipol Airport you can take a train to Leiden Centraal Station. From there you can take a taxi to Noordwijkerhout, which is about 9 miles away. Otherwise you need to get two buses from Leiden C S. (Service 40, 42 or 57 to start with, followed by 90).
Trains also run to Leiden Centraal from Amsterdam Centraal.
Bus 57 runs from Noordwijkerhout to Lisse, from where it's a short walk to Keukenhof:
http://9292.nl/#
I think I've got that all right but watch out for a post from Mushroom25, who occasionally seems to find things which I've missed!