Here the solution to run Arduino Mega and the shield W5100.
Into the file MyConfig.h, after the line:
#include <stdint.h>:
you must add the following lines:
#if ARDUINO < 100
#include <WProgram.h>
#else
#include <Arduino.h>
#endif
Then replace the lines:
const uint8_t SOFT_SPI_MISO_PIN = 16;
const uint8_t SOFT_SPI_MOSI_PIN = 15;
const uint8_t SOFT_SPI_SCK_PIN = 14;
with these:
const uint8_t SOFT_SPI_MISO_PIN = A2;
const uint8_t SOFT_SPI_MOSI_PIN = A1;
const uint8_t SOFT_SPI_SCK_PIN = A0;
Finally enable define SOFTSPI removing the comment (//)
Compile and load the software and you're done! 
