What version of CSS Sculptor do you have installed? The .pdf you linked to is for an old version, (1.1) so won't apply (completely) to later versions.
If you want to create another header within the full-width header, place your cursor inside the full-width header div, then insert a new div (Insert > Layout objects > Div Tag). For the first option (Insert) select 'At the insertion point' and give it an ID (e.g. innerHeader) and click the 'New CSS Rule' button.
In the New CSS Rule window that opens, for the first option ('choose a contextual selector') select 'ID' and then at the bottom, choose where the new rule (css) will be placed. (It would be best to browse to and select your CSSSculptor/2ColumnElasticLeftSidebarFullWidthHeader_Pacifica_css/screen.css file) and back at the New CSS Rule window, click OK.
In the CSS Rule Definition window, click on 'Box' in the left hand column list. Give it a width and height that suits the design you have in mind (966px or 88ems if the layout is the same as in the latest version). In the same window, in the section called 'Margin' click the drop-down list for 'top' and Select 'auto'.
Click OK and OK again to get back to your page.
You may want to remove the padding - if there is any - from the full-width header:
Select the FW Header. In the CSS Styles panel, in the lower section, select 'padding' and click the little dustbin (trash can) icon to delete the property.