Click here to download a zip file:
http://lame.jthz.com/lame3.97.zip
If you're asked whether you want to open the file or save it, select 'Open'. (Otherwise save it to your hard drive and then double-click on it to start your unzipping program).
Extract the file lame_enc.dll to any convenient place on your hard drive. (Exactly how you do that will depend upon which unzipping software you use. With mine, it's necessary to right-click on lame_enc.dll, select 'extract' and then navigate to the desired location). It's not necessary to extract the other files within the zip file but it won't matter if you do.
Then start Audacity. Go to File > Preferences > File Formats. Click 'Find library' and then 'Yes'. Navigate to the location of lame_enc.dll and double click on the file. If the installation has been successfully completed you should now see 'MP3 Library Version: Lame v3.97'. Click 'OK'.
Chris