DVDA 1.0: Go to File -> Properties and choose a file for Introductory Media.

DVDA 2.0: Go to the Project Overview window and click the button that has a plus sign and a purple square with a white star in it -- if you hover over the buttons, this one will say "Insert Introduction Media." Or, if you've already added the file to your project, then right-click it and choose "Set DVD Start."

If you want to ensure that the viewer doesn't skip through the introductory media, then double-click the item in the Project Overview window, go to the Properties window, and change the settings in the Remote Buttons section.

Rob