Let's say you're working with an html page that doesn't have any css rules yet. Here's a simple way to put all your content into a one-column layout that you can control a bit better.

How to center a simple one-column layout

1. Select all (cmnd+a, ctrl+a)
2. Go to the Insert menu ..>Layout Objects...>Div Tag
3. (Wrap around selection) and then click on new CSS rule
4. Use the pulldown to make it an ID, call it #mainbox, click ok
5. Under the Box category, pick a width of 500-600px (you decide)
6. Under margins, use the pulldown to make your left and right margins "auto"
7. Click OK, then click OK again.