I've never seen a graphics card with only one ADC port on. Are you sure there isn't a DVI or VGA port next to it too?
If there's a DVI, Hrududu's right, you need the an Apple ADC/DVI converter, last price retail was about £69 (UK). It's effectively a power supply, as ADC not only provides the video signal, but also power too. DVI is video signal only, hence the large brick that comes with the ADC/DVI converter.
There is a Formac version, but it's just as expensive if memory serves. Good luck!