The name XXX does not exist in the namespace YYY (XAML VS12 Windows 8)

So I'm working in VS12 on Windows 8, and hacking the ContosoCookbook code to make a different app. I'm trying to set up a "MainMenu.xaml" page and in I have:

 <CollectionViewSource   x:Name="groupedItemsViewSource"   Source="{Binding Groups}"   IsSourceGrouped="true"   ItemsPath="TopItems"   d:Source="{Binding AllGroups, Source={d:DesignInstance Type=data:FlashCardDataSource, IsDesignTimeCreatable=True}}"/>

The error I'm getting is: The name "FlashCardDataSource" does not exist in the namespace "using:FlashCards.Data".

...but I don't understand how it doesn't. Where do I start looking? I'm new at XAML.

-Ken