Camtasia cannot record games that are in full screen.
This is because Windows Aero takes exclusive control of your graphics driver.

You'd be wasting your time trying to get Camtasia to record games.
1. You can't change the frame rate at which Camtasia captures in resulting in very bad captures.
2. Camtasia isn't designed to record games. If you record your desktop and look at how jumpy the capture is, Camtasia records at (I think) 15FPS. You need to be recording at (at least) 30FPS.

If you want to capture games, You'll need to download game capture programs like Bandicam/Fraps.
But they are not free.