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)

To record audio from analogue audio equipment, you're going to need to buy a little audio interface that plugs into the USB port of you Macbook.

Here is a sampling of interfaces that should do the trick. Not all of them are cheap, but if you use it a lot, it's a good investment.

If you get one, you can plug your mixer into it using standard audio cables.