Mosaic needs to be activated in the Add-on control panel.  To activate Mosaic: 

• Near the bottom of the Toolbar, click on your User Name, then select Site Setup. 
• In the General Section, click Add-ons. 
• In the Available add-ons section, find Mosaic, then click the adjacent Install button.
  
  
• If successful, Mosaic will appear under the Activated add-ons section. 
  

Once Plone Mosaic is installed and activated, the Plone site should still look and behave normally. However, we now have access to Mosaic's layout editor.