I have version 3.2.12 installed. There is a message above the metadata fields that the metadata mambot must be installed. I read that this is installed as part of joomsef, and I should look for artio metabot or some-such in the installed components or plugins. Nothing like that there. Where is it?

Maybe it was not installed correctly then.
That may e.g. occur, if the write permissions on plugins/system directory were not set correctly during install.

The easiest is probably to make sure all permisisions are correct (all directories writable - you can check this in Help -> System info -> Directory permissions) and then reinstall JoomSEF once again.

After successfull installation, it should be display in your Plugin as a system plugin.

Would not the fact that Joomsef is working, and that the plugins list System-Artio Joomsef indicate that the directory was writable? I tested, and put an entry in the meta description of one of the articles, and it worked. I'm wondering if the message about needing the mambot (since mambots no longer exist) is outdated, and if the instructions on the site about needing a metabot plugin are incorrect, that the plugin is just the Artio Joomsef plugin.

Ok, then it is fine. You got it right.
Metabot = Artio JoomSEF plugin.

Great, thank you :-) I'll put in a request to change the message in the edit screen.