How to get a simple laptop/mp3 to jam along with?

    So I have been experimenting, googling and reviewing the Kemper reference manual for maybe two hours now and am getting very frustrated. I have a very simple set up - I plug my guitar into 'input' on the front, I plug my headphones into 'headphone' on the front, I choose one of my favorite setups (eg, Tiny Teeny Feet with some of the effects turned off) and I play. Now I would like to run MP3 files from my MacBook or youtube sound from my Macbook through the Kemper so both the guitar and what I am playing on the MacBook come through in the headphones. I simply can't figure out how to do this. Would anyone be willing to spare a few minutes to help me? Thanks.