When using the skins with tab menus, typically the tab menu only shows root level pages and pages below the root level pages are shown in the vertical sub menu. When you click the new page icon you go to page settings where you create the new page. There is a dropdown there for the parent page. By default it preselects the page you were on when you clicked the icon for the parent page. Just change it to root to make a top level page which corresponds to a tab in the menu.

Hope it helps,

Joe