The easiest way to add support for MP3 is by installing a pre-build RPM package. Fluendo has one available for free. The reason Moblin isn't distributing a decoder is of legal issues, however one might get them by "legal" ways where that term will vary amongst different juridictions. It clearly is an annoying topic to raise up! If you buy a laptop you can expect to have MP3 support since the sellers have ways to collect royalties. At least that's what I learned from the Moblin mailing-list.
Installing an RPM is done by running the simple command "rpm -i [package]" with the super user.