I am not quite sure what you mean.
The drop down button you are refering to is automatically populated with the MailMergeDataSource.ItemsSource of the currently shown document in the editor. That said, you should uncomment that line of code in your project.
I am also attaching a sample project demonstrating mail merge. The items source is set in the constructor of the window, as well as in the DocumentChanged event handler in order to be set for each new/imported document.
Let me know if I misunderstood you or you have other questions.