Kat, when you click on "format" then "page" do you get a page formatting box with numerous tabs? If so, one of those tabs should be "Header." Click on that to open the header attributes box. There should be a click-box in the upper left corner with the legend "Header on." Until you click on that box, everything else on the page is grayed out.

The reason for putting the page number field in the header or footer is that otherwise it would be treated as document text and would move around as text was inserted or deleted.