If you just want to be able to see kanji in a file list, installing the Japanese language package should be all you need to do (I think). If you want to be able to input kanji, it gets a little more complex... you can install the packages that nocturne mentioned (which are more of a 9.x solution), or use packages called XIM, anthy, and SCIM (which are what came with 10). You'll also need to switch locales, I think.
Or you can do what I'm doing... wait for 10.1. 10.1 will be using SCIM fully, and you won't have to switch locales or anything. 10.1 will have a lot of advances for those of us who use Asian characters.