I think Audacity does not ship LAME (open-source MP3 encoder) with it, so you might need to download it and enable it. It's as simple as downloading it, extracting/installing it and then specifying the path to it in the settings of Audacity. Instructions are here.
I'm also in the favour of offering the podcast in both high-quality MP3 (~320 kbps) and medium-quality AAC (~128-192 kbps). AAC will be helpful for the people on slower connections, and arguably offers better sound quality than MP3 at the same bitrate. I think the Mac people also prefer AAC than MP3 because iTunes rips tracks into it by default? AAC also allows tracks to be split into segments/chapters for better visualisation when seeking. See here and here. Anyway it's just a matter of exporting the finished file twice using two different formats. The file format is a minor point and onus should be primarily given to planning, recording and producing the podcast.
Also, as Zenith said, stereo mix needs to be enabled for most of the computers if you haven't used it before. Here is a guide to help.