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, the following pages on multilingual computing are very useful, and were helpful to me in writing these pages. They treat a variety of languages including Japanese.
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.
Yale's Chinese Mac Site, with its detailed information on Chinese support in every major version of Mac OS, has a lot of information helpful to Japanese users as well. If you want details on the differences between versions of Mac OS, or using the character palette to input kanji, this is an excellent resource.
Older Systems and Older Versions of this Site
I update this site fairly regularly 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, with the Classic Environment, etc., you might consult an earlier archived version of this site.