COM Port
The motherboard has a 9 pin RS232 header located near the eSATA ports. There's a blanking plate on the outside of the chassis where you can attach an externally accessible DE-9 connector (commonly misnamed DB-9). The BIOS redirects console I/O to the serial port, defaulting to 115200 baud. No cable is supplied. If you add one, you will be able to view the bootup process, manage the BIOS settings, boot the OS into single-user mode, etc.
Cable Pinout
The motherboard header is laid out according to the Intel/DTK style rather than the more common AT/Everex style. If a suitable cable is not available, you may be able to adapt or construct one. To avoid confusion, this line diagram uses the customary pin numbering for each type of adapter (on some DE-9 connectors this numbering is embossed onto the connector itself).
The wire coloured red on the ribbon cable goes to pin 1.
The blue lines represent connections that must be made.
Use a null modem cable to connect to a PC.
Booting
Once you've got the serial cable installed and connected to another PC, fire up a terminal program set to 115200 baud, 8 bits, no parity, 1 stop bit.
For Windows users, PuTTY(0.59 and newer) can connect to a serial port and works well, see PuTTY Settings. Alternatively you could use Hyperterminal, teraterm, etc.
For users of Linux/MacOS X/other UNIX-like OS, you have a few possibilities, including these (adapt the port name as necessary):
$ tip -115200 si0
$ cu -l /dev/tty00 -s 115200 # common on BSD-like OS
$ screen /dev/tty.usbserial0 115200 # useful on MacOS with USB serial
$ minicom
Start the unit up. You should see text scroll. Pressing DEL should get you into the BIOS Setttings. If you see text but cannot enter BIOS settings, try a couple of times in case you didn't press it at the right time, but if this continues you most likely have incorrect cabling. (As a temporary workaround if this is the case, you may be able to use a USB keyboard to enter the BIOS screens, but it is advisable to fix the cabling).