on System X Japanese is fairly easy to get going.
Go to the "International" preferences -> "Input Menu" and turn on
Kotoeri.
In the sub menu I only select Hiragana, Katakana, and Romaji.
Romaji is similar to QWERTY keyboard layout. However some of the
keys are in slightly different positions.
It's also good to click Show input menu in menu bar so you can easily
switch from English to Japanese typing.
Japanese fonts are installed by default so after that you should be
able to type kana.
While having hiragana selected as your keyboard or input method, just type the two letter romaji to get kana. ie ka becomes か
Your main problem will be stopping the kana
converting to kanji if you don't want it to. Maybe hitting "enter" then "space" after each word will work. hitting space converts to kanji.
In 10.5 Kotoeri help is in English.
JapanesePod101 has a FAQ on typing in Japanese that might help as well.
http://www.japanesepod101.com/forum/viewtopic.php?t=204
I don't use Word but I'm pretty sure it has direct input. There was a version that needed you to install support for Asian characters from the install CDs. though.
Test kotoeri is working in TextEdit if it doesn't appear to work in Word.