MS WORD in Mac OS X can have the capability to add Ruby text, as long
as you turn it on by using Additional Tools / Microsoft Language
Register (enable Japanese). Then, you can find it under Format >
Phonetic Guide ...
Mine is Office 2004. However, I haven't heard about its improvement
in Mac Office 2008. So, the situation should be similar.
The only drawback is that you have to enter them manually, one by
one. It is why I said it is inferior to Windows version, which can do
it automatically.
As for LaTeX, it is a cross-platform typesetting system. The document
content is entered with the formatting markup instructions together,
similar to the HTML fashion. It's not What-You-See-Is-What-You-Get;
therefore, it has some learning curve to go through. Wikipedia has an
entry for LaTeX:
http://en.wikipedia.org/wiki/LaTeX
Even though I create those script for LaTeX users, I am not
evangelistic for it here, because I know that many individuals do not
like to use LaTeX.