I have installed CamTwist on my MacBook (mid-2015 - Retina display).
Now every app uses that by default.
I want the default webcam to be the FaceTime HD camera installed in my MacBook.
How do I make every app use that webcam by default?
Looking at http://camtwiststudio.com/support/faq/ the following seems to apply to your situation.
PS: Development for the application may have stopped, the web site looks rather outdated.