You need to enable the SharePoint Server Publishing Features at the Site Collection level, and then also at the Site level. Enabling these features adds the option in the Ribbon.
Site Collection Feature:
SharePoint Server Publishing Infrastructure
Provides centralized libraries, content types, master pages and page layouts and enables page scheduling and other publishing functionality for a site collection
SharePoint Server Publishing
Create a Web page library as well as supporting libraries to create and publish pages based on page layouts.
Dependencies: SharePoint Server Publishing Infrastructure (Site Collection)
SharePoint 2010 | Content Publishing