Centering pages is easy with SiteSpinner and SiteSpinner Pro.
First off, be sure that you have designed your pages relative to the 'Guide Border' (icon on the top tool bar) as this is the reference guide for centering.
Go to the Page Editor > Special Effects tab, and check 'Center Page Horizontally' - that's it. If you would like the effect applied to all pages, simply press the 'All Pages' button.
A couple of notes:
a) be sure that all objects are centered within the Guide Border. The Guide Border dimensions can be changed under Options > Project Options. Whatever you set this at, your content should be placed assuming that the blue outline is the browser's horizontal boundaries.
b) do not use Relative Spacing or Relative Sizing (functions on the bottom tool bar) on any objects if you plan on using the Page Center function.