For those who has been getting the ultimate crashes from SharePoint Designer, and many weird oddities with it, this is something that you should know while working with Data View Web Parts. Occasionally, SharePoint Designer will NOT allow you to add a Data View Web Part no matter what you do, as shown below, and the main reason is... *drum roll*... your file is not saved.
The first screen shot shows a "changed but not saved file" and "Insert Selected Fields As..." not working and the second screen shot shows the "Insert Selected Fields as..." working after saving.