Problem with airport conection

I've been having troubles with the airport conection. If I put a password to my wireless internet my mac dont connect it just show me this message:

"The selected network uses an Access Control List to restrict access and this computer is not on the list.
See the Administrator for access.
Press "Try again" or "OK"

yeah I know what u are thinking... its a problem with my network and I have to contact the admin but the thing is that when I dont put the password my network works perfectly... and I have to put it cuz all people start using my internet and it becomes impossibly slow!!! I hope u can help me because I call my internet company... they send this guy that didnt know how to fix it... I live in china and I can tell u chinese people know anything about macs!!!