"/dev/mmcblk0" should replaced according to your host, use "dmesg" after inserting the SD to find out where is the SD on your host. Unmount it before issuing the dd command. seek 2, skips the first 1K bytes (2x512) of the SD where the ROM expects the boot image for SD.
5. Insert the SD on the i.MX51 EVK, and set the switches for SD card boot and power on the board.