Your initial login name and password should both be your Student ID number. You should change your password immediately on your first time logging in.
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 "Email Instructor" 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.