Using Japanese and English on the Macintosh
OSX
Using Japanese on the Mac in Os X (OS 10~) is a cinch. This is because OS X comes with Japanese already installed. It is simple.
But, how do you want to use Japanese? There are basically two ways: change the operating system and applications into Japanese, or keep the operating system in English and use Japanese in some applications.
One
The first way is to change the whole operating system to Japanese. Most of the menus in the included applications (Safari, Mail, etc.) will also change to Japanese. (Note: Appleworks will not change.)
How do you do this?
A. In the Apple Menu, go to System Preferences.
Choose "International." You will see this menu.

B. To see what languages your computer is set to use, click the edit button at the middle right.
You will see the following screen.

C. Check the languages you want to use. To use Japanese, check Japanese (nihongo). Click "Okay."
You will come back to this screen again.

D. Confirm that the languages you just checked now appear on the left where it says "Languages."
E. To use Japanese as your main language, click on Japanese and drag it to the top of the window.
F. Restart your computer. Japanese will now be the main system on your computer.
Note: You can also change languages according to log-in ID. If your computer is set up so that different people can log in, you can set one log in ID to use Japanese using the instructions above, and leave another log in ID as English. This is what my wife and I do, since she wants to use the computer in Japanese and I want to use it in English.
Two
You want to use Japanese in some of your English applications. This is very simple.
Do steps A ~ D. Then, restart the computer.
After you have restarted the computer, open TextEdit or Appleworks' word processor. At the top right of the menu bar, click on the American Flag. Your menu will look something like this picture. Choose hiragana. This will let you type in Japanese. You may need to play with this and the different ways of entering Japanese text on this menu and the input mode palette to get this the way you like it.

Last words
Now that you have Japanese set up on your Macintosh, you can type Japanese, read Japanese web pages, and so on, but aren't you missing one thing?
Wouldn't a good dictionary that you could use in your computer be nice?
I have looked far and wide for a good Japanese-English dictionary, and the best I have found is Mayjay. I liked the old MacJDict that I used to use in the old Macs better, but Mayjay is good enough for most of my needs. Here is a picture of Mayjay in action.

One important point. When web browsing in Japanese, you may need to play around with the character encoding in the browsers to get Japanese pages to display correctly. Most pages will be displayed well, but there are two or three that won't until you change the encodings on your browser.
Classic Macs (pre-osX)
The mac I use I bought here in Japan. I've kept the Japanese OS on it and it runs most English programs with no trouble. Previously, when I used my Powerbook 150, I had used OS 7 and the Japanese Language Kit, but that is a very clumsy set-up. If you you live in Japan, can read Kanji fairly well, and know macs pretty well, it's much better to use the Japanese OS with Kanji Talk. Otherwise, stick to the Kanji kit.
One of the most interesting things about the Japanese OS is that when using English programs like Explorer and Netscape to view Japanese pages, they usually display the Japanese without any problems. You just have to be sure that you've set the character set for Japanese in the preferences to do this.
In fact, in programs like Nisus, you can actually type in Japanese although it is an English word processor. I mean that the commands are in English. Nisus is set up to allow the use of two-byte characters such as those used in Asian languages like Japanese. Nisus also has a number of macros available on the Nisus Writer homepage for translators, or you can make your own. Of course, you need to have KanjiTalk or the Japanese Language Kit to use Japanese.
Another program worth mentioning is MacJDic, which basically allows you to have a pretty good English-Japanese-English dictionary on your computer for free, if you have an Internet connection. I depend on this thing. It's free and ranks up there with programs that cost much more.