I have successfully installed SCIM. What I did is as follows:
1. Install Chinese language.
2. Make sure that im-bridge-client-gtk, language-selector and scim-bridge are installed. (It seems that only scim-bridge is not installed by default)
3. Check the box at "Enable support to enter complex characters" and apply; then uncheck it and apply; then check it and apply.
4. Log out, and login.
See also: https://help.ubuntu.com/community/SCIM