WebCam Monitor detects connected video and audio devices at start up. If you plug-in a Video or Audio device after starting WebCam Monitor then you need to inform WebCam Monitor about the new device before using it. To refresh the device list to reflect changes made to your hardware configuration:

If Monitoring is in progress then on the Monitor menu, click StopMonitoring first.
If a device is already in use then stop its preview. To do so, on the Monitor menu, click StopPreview.
On the File menu, click RefreshDeviceList.

The changes you have made in the hardware configuration will be reflected in WebCam Monitor, and you can access the video/audio device you have added from the Video Device or Audio Device box respectively.