How to accept USB as audio input for New Macbook Pro Retina?
You just need a quality digital audio interface for your Mac.

The one I personally use and have had great success with has been the Behringer UCA202 Audio Interface   This installs with no drivers required El Capitan recognizes it as a USB audio input/output device.  

I have used it with just the System audio as well as with Audacity and it works flawlessly.



Since you said that you use a phono plug, you will likely need a phono to RCA adapter like the one below. Fortunately, these are relatively inexpensive (around $5 USD)