Arduino communicates with both the W5100 and SD card using the SPI bus (through the ICSP header).This is on digital pins 11, 12, and 13 on the Duemilanove and pins 50, 51, and 52 on the Mega. On bothboards, pin 10 is used to select the W5100 and pin 4 for the SD card. These pins cannot be used forgeneral i/o. On the Mega, the hardware SS pin, 53, is not used to select either the W5100 or the SD card,but it must be kept as an output or the SPI interface won't work.
Entonces tienes que modificar la librería para prescindir del CS pin.
boards, pin 10 is used to select the W5100 and pin 4 for the SD card. These pins cannot be used for