There's no ready made cable for that type that I've seen on the market. You have to make your own or have someone make it for you.
The RCA cord you used are designed for low voltage signals. Sooner or later you'll hear pops on your speakers with that cord.
You'll need RCA connectors also like these connectors
or just replace the wire on that RCA cable.
I would use an RJ45 cable if it's my installation, but speaker wires are fine. Make sure that the red connecting to your speaker is soldered at the center of the RCA connector and the ground to the outer terminal.
although, im now having problems with my auzentech x-plosion. its all fine except the volume slider bar for the digital output (ie the optical cable) is greyed and stuck at halfway. anyone know of this problem?
You can not adjust the volume of any digital signal period. However you can adjust the master volume or your Preamp/Amp or Soundcard.