If you cannot find what you need on this site, here are some additional places to look.
If you have trouble following the directions on this site and need step-by-step guidance, one alternative is YouTube, which has some instructional videos created by users that show you how to enable Japanese input, for example.
Another alternative is the Mac's own Help function. If you select the Help menu from the finder and enter a search phrase like "Typing in Japanese," it may be able to walk you through the steps necessary to activate Japanese input.
More Detailed Information
For more detailed information, Tom Gewecke's epic Multilingual Mac blog includes copious detailed information on non-English language support on Mac. Some of the tips and tricks on my site come from this blog, and it has more information not included here--particularly about changes to language support in the most recent releases of Mac OS.
Older Systems and Older Versions of this Site
I update this site periodically and remove references to earlier systems. The advice on this site should be helpful if you have a fairly recent version of Mac OS X. If you need information about using Japanese with an older version of Mac OS, you might consult an earlier archived version of this site.