Using SharePoint designer 15 you can develop workflows that’s identical to SharePoint designer 2010 workflows or target the new workflows in SharePoint 2013. This is dependent on the platform type you choose when you create your workflow (see the following screen shot).
Sometimes when you try to choose the platform type you will see only SharePoint 2010 workflow option. to be able to use SharePoint 15 Workflow option you will need to have Windows Azure workflow installed and configured.
Theses are helpful resources