when there are line/phono switches on one channel, I put serato on line and the turntables on phono.
There will be two sets of outputs from the Serato box, right? One is the throughput from the turntable, and one is the output from the computer. Set it up as directed and make a note of where the "through" cables are going in to your mixer - that's the channel you have to set your output to in order to play it.