Well you don't use <center></center> at all for anything, really, because that's not even
real HTML.
And you don't "center a page in XML" because XML, in general, describes data rather than layout/formatting, so doesn't have a concept of centering.
To give you a useful answer, I need more information.
What are you actually using - is it XHTML, is it file.xml with a related file.xslt, or is it something else?
Do you mean center like this:
I am centered text
in a left-aligned box.
Or like this:
I'm a centered box,
with left aligned text