Audacity mp3 tutorial
Here is a link where you can download the LAME mp3 encoder you need to do this.
link: http://audacity.sourceforge.net/help/faq?s=install&item=lame-mp3
click on the .zip file and save it.
open it and withing you should find a file called lame_enc.dll that is what you need. when audacity asks you what where it is, find that exact file and then just double click on it.
