Using USB Thumb Drive

I've been trying since we purchased our car, to be able to use a USB thumb drive to play music in our 2015 Cadenza, but to no avail. I'm guessing we're missing a manual, because I don't see anything in any of the documents/manuals/pamphlets that came with the car as to how to do this. I know it works, because the sales guy (long gone from the dealership now) used it when we test drove the car. I'm guessing its a formatting issue, but I'm clueless and don't see anything in the forum that details it. I've tried just moving the whole folder over from my laptop onto the thumb drive (all music folder, with sub folders for artists, then sub folders from there on albums). No go there. Also tried pulling over just a single album, but no go. the files are just standard MP3. Any guidance here would be appreciated. Thanks in advance.