Add a bass track to an imported mp3

I have a Presonus mixer. Connected to the iPad with the camera adapter. I can hear the bass through my amp but no signal goes into the iPad.

I did set the new track to Channel 1 (from the amp) but still no signal. Does anyone have a step-by-step guide or a good video to watch?

I want to add a bass track to one of my band member's mp3 file. Thanks. Pete