@Tom > That's a diagnostic port, completely useless to anyone but the engineers.
As for how to use Serato with the computer, you'd simply connect the CDJs to the inputs on the SL box, then the outputs on the SL box to the inputs on the mixer. You'd run the timecode CDs in the players and that would control the software. That's it - no extra plugs, only the SL box connects via USB to the computer.