Click GO LIVE in Procaster.
Once you are live you'll see a Procaster dock with icons on the bottom of the stream.
On the lower right:
CAMERA, SCREEN, 2D MIX, 3D MIX, ZOOM OUT, ZOOM IN, MINIMIZE.
Click on the word SCREEN
Click on ZOOM IN to stream a targeted portion of the screen.
You will likely get much better quality from Procaster than you will from CamTwist.