Jim:
I simply go to the plugins:encoders page, and shift-click the download link for each plugin. On the others the plugin file is recognized as a Mediacenter file, and I choose the option to open in Mediacenter. Then the plugin installation starts and I click "Ok" a few times. With the mp3 plugin I choose "Open with Mediacenter" and it immediately crashes.
Ahh... There is absolutely no need to do it this way, and possibly it isn't supported anymore (though Matt should probably check on this).
If you want to encode to MP3, you can install that plugin through MC itself (not using your browser). Open MC, go to Tools --> Options --> Encoding. Then next to
Encoding For choose CD Ripping (if that's the type of encoding you want to set). Then down below next to
Encoder select "MP3 Encoder". This will download and install the correct LAME plugin for your version of MC (if you don't already have it). Then you can click on
Advanced and pick VBR/CBR/Quality/Bitrate and all that good stuff.
If you also want MC to encode to MP3 when you are Recording Sound (from your line-input) or Converting Files you need to select those options under
Encoding For (again, at the top of the window) and configure those separately.