Setting Up My Audio Devices

I recently purchased Cubase artist wanting an upgrade to something more professional after using garage band for a few months to learn the basics. Please understand that I’m very new to all this. I’m having trouble setting up my audio devices. I have been using a Rocksmith cable for my audio input and it was working fine with decent audio quality on garage band. However, on Cubase, i seem to be unable to use this setup. No matter what I do, it only seems to pick up the built-in mic on my mac. I tried setting up an aggregate device but I still couldn’t see anything other than built-in audio in the “Audio devices” window. What am I missing?