This is the top reasonable result for a google search of "irig macbook pro" and here's what finally worked for me:
- reduce strain on iRig by guitar cable etc hanging off it
- push it in and hold it for a bit in place to give Macbook Pro time to recognize it, you'll hear a little glitch of sound when it happens
that's it
PS irrelevant but perhaps useful stuff :
it was hit & miss for me until I did this last night finally, it had been a month without iRig connected (was using Rocksmith USB cable instead). It was a clean install of El Capitan and I couldn't use my iRig that had been working after some fiddling on Yosemite (MacBook Pro late 2013 which can use 3-stripe input/output miniplugs like the iRig for both input and output).
Another fun tip, hold down option before choosing volume when it is in menu bar (enabled via checkbox in sound settings panel) and choose which of your inputs/outputs to make active, including any ones you made in audio midi utility