I'm using NOF10. When previewed, the page contents are shown on the left which is how it is by default in design view. The only way I know is to drag all the contents and objects one by one in to the middle of the page, but of course this is very time consuming and not accurate.
For example, PC Advisor, the Guardian, the Times have their page layouts centered. (I think most sites do - the big ones anyway)
In NOF, you can change your layout to a dynamic layout from the Tools menu (Site Options?), which then allows you to check the option for a centred layout on the properties palette. That's how it was done waaaay back in NOF 8, anyway -- I dropped WYSIWYG a long time ago, so I'm working from a very hazy memory. If you're building a table layout instead, then you would make a 100%-width table with auto width for - say - the leftmost and rightmost columns. Then they can resize dynamically while the remaining columns do whatever you set them to do.
But tables are not good things to be building websites with.
In case anyone is interested, the modern CSS way is to specify auto left and right margins for your main parent div if you're using a fixed-width layout, or to specify % widths for the divs in a dynamic layout.