Use your UCSD (ACS) e-mail account username (use jsmith, not jsmith@ucsd.edu) and password.
If you don't know your UCSD e-mail account information, you can find it in your registration packet or use the ACS form to look it up.
If you're a graduate student and use a department e-mail server, register online for an ACS network account, then use the username and password to log in to WebCT.