Load the below code and open the serial monitor to see if the router assigned an IP address to the arduino.
#include <SPI.h>
#include <Ethernet.h>
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
void setup()
{
Serial.begin(9600);
// disable SD SPI if memory card in the uSD slot
pinMode(4,OUTPUT);
digitalWrite(4,HIGH);
Serial.println("Starting w5100...");
if(!Ethernet.begin(mac)) Serial.println("failed");
else Serial.println(Ethernet.localIP());
}
void loop() {
}