After you accept the offer, there are some behind the scenes IT processes that create your student account details. This usually takes up to a few hours before those details are emailed to you.
See
http://sydney.edu.au/start/accept-your-offer-and-enrol.html#enrol
Usyd had some IT stuff and fixing going on yesterday which delayed this process, but it might be finished now.
Have you been sent your details now.
Please note BOS is not the official usyd help desk.