Looking at w5100.h:
private:
#if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
inline static void initSS() { DDRB |= _BV(4); };
inline static void setSS() { PORTB &= ~_BV(4); };
inline static void resetSS() { PORTB |= _BV(4); };
#elif defined(__AVR_ATmega32U4__)
inline static void initSS() { DDRB |= _BV(6); };
inline static void setSS() { PORTB &= ~_BV(6); };
inline static void resetSS() { PORTB |= _BV(6); };
#elif defined(__AVR_AT90USB1286__) || defined(__AVR_AT90USB646__) || defined(__AVR_AT90USB162__)
inline static void initSS() { DDRB |= _BV(0); };
inline static void setSS() { PORTB &= ~_BV(0); };
inline static void resetSS() { PORTB |= _BV(0); };
#else
inline static void initSS() { DDRB |= _BV(2); };
inline static void setSS() { PORTB &= ~_BV(2); };
inline static void resetSS() { PORTB |= _BV(2); };
#endif
They don't seem to have a define for the Atmega1284 and the default would appear to be incorrect for you.