Sorry Cherie - I just had this vision of Simon at the hospital.
This is a tad confusing on how it works. You knock the first time - which gets Simon to remark that he needs to enter the secret code.
The one you get from the boys in the alley after you make them a new game. If you haven't done that yet - you need to do that first. If you have and you have the secret knock - you knock once - get the comment and then immediately hit the ctrl/action key in the knock pattern.
For example if it was 3 short knocks and one long one - you would hit the control key 3 quick times - pause for just a sec and then hit it again. Then wait and you'll hear the pattern.
Try it on something short and then you'll quickly figure out how to do it and it won't take much longer to get the real secret knock entered. Once you do it and get in - you never have to enter the knock again. After that you when you go back you automatically enter the club house.