Do yourself a favor and install scim. It's a wrapper program for various input methods. It's much more user friendly than working with kinput2/canna alone. When I was using kinput2/canna I had to use a script to change the environment to Japanese every time I wanted to use Japanese in a program. But scim allows you to keep a full native-language environment while also allowing you to have foreign input support. You can use canna with it, but there are several other backends available. I like Anthy myself.
The real only drawback, if you could call it that, is that you will have to convert your system to a utf8 environment (if you haven't already).
See the link below for more. Especially look at section 6.4 about input methods.