It's not going to work without modification.
You can see this right in enc28j60.c, which clearly does not match your setup
#define ENC28J60_CONTROL_CS 10
#define SPI_MOSI 11
#define SPI_MISO 12
#define SPI_SCK 13
Anyway, the state of the art ENC28J60 library is EtherCard, although that will require modification to work with Mega also.