Mixxx is not available directly from Fedora because Mixxx statically links to libmad in order to support mp3 files.
Fedora is unable to include encoding and decoding support for the MP3 format because it requires patented technologies and the patent holder has not provided licenses that are compatible with Fedora's requirements source
Instead Mixxx is available from RPM Fusion.
Enable RPM Fusion using the instructions here:
su -c 'yum install mixxx'