Under Camera Settings, in the left-hand explorer, click the red camera button to switch the camera on. From devices drop-down menu, select the camera you need to use, if you have more than one connected to your computer. Remember to enable all your USB Capture Devices in Windows Device Manager.
The problem usually happen when you have a third-party software controlling or interfering with your webcam. Some tools like ManyCam doesn’t allow the recorder to preview or capture the webcam layer. Please remove if you have any of these tools. Please any other camera, update the camera driver, and double check that no other applications able to use the camera – or is it only PresentationTube recorder having the problem?
Also, try to reboot the system. Sometimes when cameras are installed, they get set as the default camera, displacing whichever device was previously the default. If there are applications running on your system that have previously enumerated the cameras, installing a new one may confuse them. If this were the case, I would think a reboot could help.