Pay attention to emails mentioning Software Asset Management (SAM) -- yearly, you need to scan your office computer, otherwise your IP will be blocked. (If you cannot access the internet, it might be the case: http://hkusam.hku.hk )
Online course selection (on HKU Portal) is done in August for Semester 1, and in December/January (check the academic dates -- the link is above) for Semester 2 and Summer Semester. You may change your courses later before they start or during their first weeks using a paper form: http://www.gradsch.hku.hk/gradsch/web/student/course/selection.htm
The whole course selection process is first-come-first-served, so adding courses later may be a matter of luck.
If you wish to stop receiving so many HKU Junk Emails, you can remove yourself from irrelevant mailing lists on HKU Portal: http://www.its.hku.hk/documentation/guide/communication/bulkmail/recipients
(Actually, a lot of things can be done on HKU Portal, but they are often quite hidden -- luckily, it has a fulltext search function.)
Moodle (as of 2016) only displays your courses from the past two years. If you wish to access other courses, you can do so on the archived Moodle servers: https://intranet.cs.hku.hk/moodletools/MyElearning