LAME encoder to export MP3 files with Audacity.
Go to the LAME download page.
Under "For Audacity on Windows", left-click on the link "libmp3lame-win-3.97.zip" and save the zip folder to anywhere on your computer.
When you have finished downloading the ZIP folder, unzip it and save the file lame_enc.dll that it contains to anywhere on your computer.
The first time you use the "Export as MP3" command, Audacity will ask you where lame_enc.dll is saved.
In case of difficulty, please view the Audacity Wiki.