I thought the message box area was centered by default?
Any how the templates your want to be looking at are
newreply and
newthread in these the td with the class 'panelsurround' should be align center:
Code:
<td class="panelsurround" align="center">
<div class="panel">
Let me know if thats any help, I've got a sneaking suspicion that the 'Message Area Width' has been set to 100% on your style (default being 540px).
Thanks,
Mat.