Go here, hope that helps:
https://a4.ucsd.edu/tritON/Authn/UCS...auth%2Fstudent
If that doesn't work, follow this list:
1. go to
UC San Diego Home Page
2. click on the 'Current Students' tab.
3. once there, at the top of the page it should say "Announcements and Deadlines" in a green box.
4. Under this heading, it should say something like "winter enrollment times are now available" with the "winter enrollment times" leading to a link (which I supplied above) that prompts you to enter in your PID/username and password.