MP3 option doesn't show up in file menu

How do I make a MP3? The MP3 option doesn’t show up in the file menu. Where do I find it?

Download and install the LAME MP3 encoder from [u]this page[/u].

Instructions for installing LAME are here:
Once you have LAME installed you will be able to export as MP3 by selecting “File menu > Export” and then select MP3 as the file format in the export dialogue screen.