Import music files on your mobile device to rekordbox (iOS) first, then connect the mobile device with the CDJ-3000 and play the files on the unit.
You can also export music files from rekordbox (PC/Mac) to rekordbox (iOS) and play them.
You can look up the rekordbox (iOS) system requirements and supported music file formats [here].
*Audio files downloaded from Apple Music, etc. can't be used.