If you want to be able to import audio in the MP3 format (there may be other formats other than MP3 that are supported), make sure you select the non-free version of Audacity in the Software list, named:
audacity-freeworld-2.0.0-1.fc18
as opposed to the free version I installed named
audacity-2.0.2-2.fc18
Correcting my mistake of installing the free version of Audacity, I first suspected to add MP3 codecs support in my system doing this (gotten from a DuckDuckGo search):
su - yum install gstreamer-plugins-bad gstreamer-plugins-ugly
Yes, it did find some packages to download, and yes, it did install. This probably worked because I already installed the RPMFusion package beforehand (see my earlier post).
But it didn't fix the problem: going back to Audacity and attempting a MP3 import still produced the error message, "This version of Audacity was not compiled with mp3 support."
This fixed the problem, based on a tip found in the Fedora Forums.
su -
yum remove audacity yum install audacity-nonfree
