Just wrap whatever you type with (center)(/center) tags.
CODE
[center]yo wassup??[/center]
yo wassup??
Or you can think of it as Microsoft Word, and just click the "center text" button. It's like in the top-right corner of the interface which is accessed by clicking add reply.
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Wait, nvm. That may not have been what you were asking. Could you say your question a little more clearer?