Design time data is not available on all platforms. Windows Store App don't have the Data panel available in Blend for Visual Studio 2012. The XAML namespaces for creating sample data are available, so you could create design time data from code. Have a look at this to tutorial on how to do that.
It won't be of any help working with Blend for Visual Studio 2012, but in the next version of Blend support for design time data is added.