This is more of a general Mac question.
On the Mac, you can perform a screen shot by hitting Command-Shift-3. But that takes a screenshot of your entire screen.
So you can hit Command-Shift-4 and that give you a nice crosshair where you can click, drag and release to capture a rectangle from your screen.
However, what I do to take screenshots of the simulator (all the screenshots for the book are taken in this manner) is I hit Command-Shift-4 to give me the crosshair. Then I hit the spacebar, and the cursor turns into a Camera icon. You can click on any window that is currently visible once in the Camera mode and it will take a screenshot of that window. Then, I crop the image in Preview to get rid of the iPhone device surrounding it.