Fedora won't ship MP3-capable software because the Fraunhofer Institute's patent license terms are not compatible with the GPL.
You can download plugins from from gurulabs or can search for
xmms-mp3-1.2.10-16.i386.rpm in google..
To install xmms and make it MP3-capable, start by doing this:
yum install xmms xmms-mp3
You can also search for Fedora Multimedia Installation HOWTO at tldp