Ideally, you want to go digital out of your R300 into your computer...or else you're going D/A>A/D. If you've got S/PDIF out, you'll need a soundcard with S/PDIF in. If you don't have one, there's a bunch of good recommendations elsewhere in this forum. I personally use the Egosys U2A but there are cheaper bit-perfect options out there...
Edit: So to answer your original question, you probably want soundcard help, not cable help.
What kind of soundcard are you running? If you're not getting a signal, but you do have S/PDIF in on your card, try fiddling with the clock/sampling settings...I'm sure someone here will be able to help if we know what kind of card you have.
--Dave