How can I listen to MP3 Files in QC?

When I travel, I would like to take just QC, my guitar and my Ipad or Iphone. How can I listen to my guitar and MP3 files in same time on QC ?

Since the QC is USB audio compliant, you would simply plug your iPhone directly into the QC (you would likely need a hub or adapter cable depending on which setup you have). Many do this frequently for backing tracks and related.

I know it works for midi files, but I need it for MP3 and I don’t know how to do

It works for any audio file. Plug your iPhone into a hub and connect your QC (or get an adapter cable that includes the applicable ends). Start playing Youtube or something related etc. Pull your display down to indicate your I/O page to ensure you are seeing USB audio signal. The QC is core audio compliant so this will work by default whether its a WAV, MP3 etc.