Take good look at the link I posted. Right next to the photo of the header, TJ writes:
The voltage levels are 3.3 volt CMOS (Complimentary Metal-Oxide Silicon). I connected the serial port to a PC using a CMOS to TTL (Transistor-Transistor Logic) MAX3232-based converter and a serial-to-USB MCT-U232-based converter.
You can get one on the net, e.g. ebay, for a few quid. I use one from sparkfun electronics and have had no problem with it.