THESE PEOPLE ARE HAVING A LAUGH, literally at your expense!
You've paid for and own the best computer and operating system in the universe, now get to know how to use it, and SAVE YOURSELF SOME MONEY!
Just press COMMAND+SHIFT+4, Left Click and hold, then drag out the area of the screen you want to capture, let go of the mouse button and - Hey Presto!
If pressing three keys simultaneously presents a challenge, those king people at Apple have provided an alternative for you, just look in your 'Utilities' folder, there you will find an application called 'Grab'