1. Put your cursor in the first paragraph using the heading style that appears in the document. If there is no such paragraph, create one. Let's use Heading 1 for this example.
2. Using the List button to the far right of the formatting bar, or the List tab in the Text inspector, set the numbering style you want for the heading.
3. In the style drawer the downward arrow to the right of the Heading 1 style. Click on it and select "Redefine style from selection."
4. All heading using that style will now be numbered. If you want to change the numbering style, go to the FIRST APPEARANCE of the heading style in the document.
NOTE that this only works for first level numbering. Say you want Heading 2 to display the second level of a multi-level list (e.g., I.A.1.a.(1).(a) . . ) Pages will only allow you to set Heading 2 to the first level of the list. To make Heading 2 display the second level, you'll have to manually tab the paragraph over.
Good luck,
Terry