The serial port is a 2x5 pin connector (labelled CN4) that is located behind the front cover. The pin layout is shown in the picture below. In order to connect to this serial port, you need a 3.3V TTL to RS-232 level shifter.
Please note that you can easily access the serial header without voiding your warranty by taking off the front plane. Once you have connected your serial console, you can use a terminal emulation program to connect to U-Boot, the boot loader used by this device. You have to connect with 115200 baud and set the parameters to 8N1 (8 data bits, no parity, 1 stop bit).