That shield uses SPI. On the UNO, that is on pins 11, 12, and 13.
SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication using the SPI library.
On the Mega, that is on pins 51 to 53.
SPI: 50 (MISO), 51 (MOSI), 52 (SCK), 53 (SS). These pins support SPI communication using the SPI library.
Note that the CS pin is not necessarily the one you want to use, but that the pin MUST be set to OUTPUT, even if it is not the pin you use.
You'll need to jumper from the hardwired pins the shield used to the correct pins on the Mega.