For Audacity to export MP3 files you need to point the MP3 Library to the correct Lame plug-in. Directions follow.
1. Open Audacity, select the Audacity>Preferences menu
2. Select Libraries. Click on MP3 Library: Locate. (No need to download anything - The Blake School image already includes the Lame MP3 Library).
3. Click on the Browse button. Browse to the /Applications/Audacity/plug-ins folder and select libmp3lame.dylib (make sure you go to the main Applications folder not your home directory Application folder). Once selected click OK. You can now export MP3 files.