See http://www.math.wustl.edu/~blake/wwdocs/students/password.html for more password and username information.
Between the open date and due date, you can work on problems and submit answers to be graded and recorded. After the due date, you can work on problems and submit answers to be graded, but your scores will not be recorded.
To report the error, scroll down to display the Problem Set Version Number (PSVN). Make a note of this number, and the problem you are working on in the set. Click the "Feedback" button to open an email form. In comments, put the PSVN and problem number. Enter your email in the appropriate box, scroll down and click "Submit Your Comments".
You should also speak to your instructor or T.A. about the error.