The Image page in the Main Window allows you add or delete videos in the current project, the same way you would add or delete still images in a digitizing project.
If you are currently in a digitizing project, you need to use [File|New] menu in the Main Window to reset Photo4D before you can add a video. When adding the first video to your project, a dialog box called Image type will appear, asking you to select an image type. Select "Video" for a motion capture project.