djm 3000 and laptop setup help.

Hi

I have bought a pioneer djm 3000 and a USB interface and having trouble working out how to connect it to my laptop to run serato.

I know from laptop to USB interface to use USB cable. Then have speakers connected to USB interface output. 

And that's about where I'm at. The input from interface if I plug into channel one only let's me use the one channel. I want to be able to use 2 channels. And can't get serato to recognize djm. 

Anyone that has any idea or tips. Let me know.

Cheers