Connecting Altec Lansing ADA885 speakers to the Mac Mini

I would like to use Altec Lansing ADA885 speakers with my Mac Mini. The ADA885 uses S/PDIF digital input, and supports AC3 & PCM. The system came with a S/PDIF connection cable terminated with RCA-type plugs. In addition, a splitter dongle was provided which has one end terminated with a 3.5 mm pin plug and the other ends with RCA-type jacks (for either AC3 or PCM).
I tried connecting the dongle's pin end to the Mac Mini's (digital output) using both the AC3 or PCM configurations...but neither configuration resulted in any sound at the speakers. (I plugged in a pair of Bose headphones & confirmed that I was getting sound from the Mini.)

Is there another combination I can try, or do I need a different cable/dongle to get this to work?

Mac Mini Mac OS X (10.4.7)