It is a standard cable that you will need with a 3.5inch jack on either ends.
One end goes in to the Minidisc's Line out socket and the other into the Sound card's Line In. Make sure you do not use the Microphone socket on your soundcard as it will likely distort.
This is what you need:
They cost about £1.20 for 1 metre from cableuniverse.co.uk