Doctor appointment: http://www.uhs.hku.hk/appoint/appointment-m.php
Also, note only HKU students / staff can make use of UHS -- if you have a non-HKU visitor, you need to go outside of the campus (to either public hospitals or private doctors, e.g. on Queen’s Road West).
Directly on the campus, the University has its own health services for students and staff members. They, however, only have General Practitioners (GP) and Dentists. If you need to see a specialist doctor, you need to go outside of the campus (e.g. to Queen Mary Hospital or Sai Ying Pun Jockey Club Polyclinic). Once you have HKID, the prices in public hospitals (as of 2013~2015) are HKD100 for the first visit / registration, HKD60 for subsequent visits, ~HKD100 for ambulance, ~HKD150 for a day in a hospital. Note that the appointment with specialist doctors may be hard or impossible to get due to overcrowded HK hospitals -- one advice is to first go to see GP at UHS, he/she will write you a letter which will make it easier / possible for you to get an appointment with a specialist doctor. Even so, you may still need to wait several months or even a year. If that is unacceptable, you may need to go to see a doctor in your hometown or see a private doctor (which may be quite pricey).
One final word of advice -- emergency contraception pills cannot be bought in HK without a doctor’s prescription.