Are you sure you've got the camera plugged into the video in and not the video out? Make sure all the cables are where they're supposed to be, but it sounds like to me that you'd be better off using Firewire if your DV cam supports it
However if its not a DV cam (You said playing tape which makes me wonder if its an old style CamCorder), you may need to start installing different drivers and the like to get this to work