Macetech's example code uses the Wire library, and from the documentation:
On most Arduino boards, SDA (data line) is on analog input pin 4, and SCL (clock line) is on analog input pin 5. On the Arduino Mega, SDA is digital pin 20 and SCL is 21
..and that just leaves you with VCC and GND to hook up.
I'm not aware of any libraries for the DS3231.