The Style names are "Heading 1", "Heading 2", etc., not "Header1", "Header2", etc.

You should be able to apply the "Heading 2", etc. Styles via the Styles panel on the Ribbon's Home tab.

If you can't see them there, the simplest fix is to click on the little down arrow in the bottom right corner of the Styles panel, then from the Styles window that opens up, click on Options>Select styles to show>All styles. The Styles window should now list all Styles.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]