UPDATE Made it a community wiki in case someone else has something to add, I can’t keep this updated right now 🙂
It could just be that you didn’t activate the camera for the emulator. Opposed to the stock emulators that once you set up the webcam they just work, on genymotion emulators you need to turn the cameras on. The process is very simple.
On the emulator window, press ctrl+3 or ⌘+3 to bring the camera configuration panel that looks like the left, and turn it on like on the right:
You can select a dummy camera which will show a genymotion logo moving around the screen or use the system camera if it’s available and preview it. Note that on the above screenshot I’m using the phone front camera instead of the back camera.