SiteCam may be having trouble finding your digitizer. SiteCam requires a video digitizer such as a QuickCam or other video input. It must be QuickTime compatible and be fully installed. You should be able to see video with the software that came with the digitizer (such as Apple's Video Player or the QuickCam software.) But make sure that no other applications are using the video when you start SiteCam.
Check the input for the digitizer in the "Configure Digitizer" in the File menu. Most digitizers can be set to more than one input--for instance, s-video, composite or TV. QuickCams can be set to Modem Port, Serial Port.
The monitor settings may be incompatible with grabbing video. You can try changing your monitor to capture at thousands of colors. These settings are found in the Monitors & Sound control panel.
The digitizer may not be using the right method of capturing video. Go to the Fine Tuning section.