how do i fill data on a c wpf?

Try to set the Content of the window to a FlowDocumentScrollViewer that displays your FlowDocument:

Content = new FlowDocumentScrollViewer() { Document = flowDoc  };

Then you should see the document.

The other option is to create the FlowDocumentScrollViewer in the XAML markup of MainWindow.xaml and set the Document property of this instance:

<FlowDocumentScrollViewer x:Name="fdsv" />

...
fdsv.Document = flowDoc;