Usually the best places to find evening courses or for adults is at Universities (see section School System).

You have for instance the Chinese Language Centre at the Chinese University of Hong-Kong for learning languages and especially Cantonese and mandarin:
www.cuhk.edu.hk/clc/new/en/

Don't forget to check out the Open University in Hong Kong as well which are specialised in distance learning degrees:
www.ouhk.edu.hk

For other types of courses (languages but also cooking or fitness classes), see the English Speaking Members Department of the Hong Kong YWCA:
www.esmdywca.org.hk

For more professional courses if you would like to change your speciality or learn a new trade or skill, check the Employees Retraining Board (ERB):
www.erb.org/Corp/home/news/en/