Lame Library needs to be installed additionally to export mp3 files from audacity.
Use the current Audacity and download Lame Library v3.98.2 for Audacity.dmg.zip
When you have finished downloading, extract and double-click the .dmg to mount it, then go to the Finder (in Safari, “Lame Library v3.98.2 for Audacity.pkg” will be extracted automatically after downloading).
Double-click “Lame Library v3.98.2 for Audacity.pkg”. This will install the LAME binary “libmp3lame.dylib” in /usr/local/lib/audacity
If Audacity asks where the MP3 encoding library is when you export as MP3:
Click Audacity > Preferences then the Libraries tab ## Look for the MP3 Export section
Click the “Find Library” button
In the dialog box, navigate to /usr/local/lib/audacity, and select “libmp3lame.dylib”
Click Open, then OK