Please fully quit WebCam Monitor (Quit from the WebCam Monitor menu or press Command-Q on the keyboard) so that it does not show up in your Dock. Then try opening WebCam Monitor again.

Please also make sure that the camera is directly connected to your Mac; not to a USB hub or through the keyboard. If you change the USB port the camera is connected to, please make sure that the WebCam Monitor application is fully quit as above and then reopen it after reconnecting the device.