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What version of Mac OS are you on? You should be able to select Chinese simplified in the language preferences..

无想手中国人, 好把?
 
If you're using Snow Leopard, go to System Preference > Language & Text > Input Sources. From there you can select what type of chinese input you wish to use, go to the section of Chinese - Traditional and there are various choices... Zhuyin if you know, Pinyin - Traditional, etc.
 
I'm using Leopard 10.5.8 and I can't type any chinese character (or any other language by the matter, except from English and Spanish) even though it appears like selected. In the input menu bar the flag appears opaque. Can someone help me as well?
 
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