Q: center a document in Numbers

Take a look at Ians (Yellowbox) user tip:

Print Layout Guide for Numbers 3 on Mac

You can also try placing a rectangle that fillss the page, then scale and center the content on the rectangle.  Then when you print scale the content so the rectangle fills the page.  before printing, you may (if you want) remove the rectangle

I agree.

You can still use Numbers 2.x.  If you had it before there is a good chance it is still on your computer in the folder "/Applications/iWork '09"

Two things:

1. In Numbers 3.x, while you can't work in page layout (bummer--way to go, Apple), you can still show rulers. Pages are measured in printable areas. So if, for instance, you're using U.S. Letter with 1" margins, you'll want to be aware your layout will tile across at every 6.5" (6.5", 13", 19.5", etc.). It usually doesn't make much sense to worry about the vertical run-off, but with 1" margins as above, that would mean a new page at ever 9". Table headers should still be reprinted on those vertical run-offs.
2. Not to scold, just to be helpful: if someone helps you with an answer, you should mark their remark as the one that solved your problem. For instance, in Wayne's comment (where he says, "Take a look at Ians (Yellowbox) user tip…"), that's the remark that helped you. Those points under his name indicate how many people he's helped. If he helped you, you're cheating him points by (what appears to be) patting yourself on the back when you mark your own response as "This solved my problem." Your comment didn't solve your problem, his did, so his should be checked. I'm sure you didn't mean to do that, just letting you know. (Wayne's probably too nice to point this out, but since I don't care if I come across as a jerk on behalf of someone else, I don't mind as much. )