To center your pages in Freeway select the Master page or page you want centered and select Center from the Align option in the Inspector palette.
Remeber, if you have a background image on the page then this will also need centering too. To do this go to the Appearance Settings panel of the Inspector palette and choose Center from the Horizontal option. Leave the Vertical option on Top or Tile though as vertically centering the page in the browser in the same way isn't possible so the page and the image won't match up.