scim is only a frontend. You also need to install an IME program like anthy or prime, plus the related anthy-scim, prime-scim, etc. packages. That's the nice thing about the scim system, you can switch between many different input editors.
Here's a good page on International text support. It focuses on Debian, but I'm sure you can adapt it to your system. http://www.jw-stumpel.nl/stestu.html