Problem with Ethernet module on Arduino uno

Hello.

I tried this with my uno and it works. But when i try it with my Ethernet shield attached it does not work.

Can anyone tell me why is that?

#define SS_PIN 10 // Configurable, see typical pin layout above

That's your problem. The Ethernet Shield uses Pin 10 for its SlaveSelect. It also uses Pin 4 for the SD card Slave Select. You will need to move the RFID Slave Select to a pin that the Ethernet Shield is not using.