You are using a very old version of the Arduino IDE. Since the Arduino AVR Boards hardware package used by the Arduino Mega is bundled with the Arduino IDE, you are also using a very outdated version of Arduino AVR Boards. In more recent versions of Arduino AVR Boards, Arduino updated to a new compiler version. That compiler update caused some bad code that compiled with the old compiler to no longer compile. In order to provide backwards compatibility, Arduino added the -fpermissive compiler flag, which caused the old bad code to continue to compile. As a side effect of -fpermissive, it allowed even more bad code to compile than ever before. The pushbuttonlinein sketch you found contains some of this bad code, and thus will only compile if you are using a modern version of Arduino AVR Boards. Although it is probably possible for you to just update Arduino AVR Boards via Boards Manager, I would recommend that you go ahead and update your Arduino IDE instead, unless you actually have a good reason for sticking with Arduino IDE 1.6.5.
You can download the Arduino IDE here: