I can tell you that it does compile for ESP8266. I suspect that the author didn't write the library for AVR or if they did they have a modified AVR hardware package.
I believe if you copy the original library code directly from Arduino Playground - SimpleTimer Library rather than the modified version of the library at GitHub - schinken/SimpleTimer: Fork of Arduino SimpleTimer Library (http://playground.arduino.cc/Code/SimpleTimer) you will find it compiles fine for your Mega 2560.
It's unfortunate that schinken didn't make any effort at documenting the limitations imposed by their modifications.