Installing plugin 'simplepie.inc' after having visited 'Feed importers page' gives 'Missing plugin' message.

Can't reproduce. This needs a better description on how to reproduce the error.

What are "feeds options" - the Importer settings on the admin/build/feeds page?

I guess I have the same problem, but maybe I'm missing something:
-Installed Feeds
-Placed simplepie.inc in modules/feeds/libraries
-No new options show up in "Home › Administer › Site building › Feed importers" when editing a Feed Importer and selecting a Parser

I did either 'empty cache' or 'uninstalled' a module to get the new parser into the list.

When installing the simplepie.inc file _after_ having tested the feeds module does not make feeds discover the new 'working' plugin.

My guess is there is a caching problem somewhere in the feeds / ctools pipeline.

Workaround:
Enable, Disable then Uninstall a not used module ie color.module or through devel.module

See #778416: Better message when plugin is missing for a somewhat improved message to know which plug-in is malfunctioning.

Reading up... I am not sure but does clearing the cache fix or not fix the problem of simplepie not surfacing?

Would this modification to the README cover the problem?

- To use SimplePie parser, download SimplePie, place simplepie.inc into
  feeds/libraries, go to admin/settings/performance and clear all caches.
  Recommended version: 1.2.
  http://simplepie.org/

I'm not sure. I suspect ctools to cache the compile/parse result of FeedsSimplePieParser.inc as failed and will not recover even when clearing caches.

According to #778416-5: Better message when plugin is missing the 'Clear all caches' should work. Maybe that link to admin/settings/performance should be added to that message.

What's going on is that as of recently we cache plugin information.

See:

function feeds_ctools_plugin_plugins() {
  return array(
    'cache' => TRUE,
    'use hooks' => TRUE,
  );
}

Next steps to go to the bottom of this problem:

a) Verify whether clearing cache through admin/settings/performance clears CTools caches (if yes, #5 is a fix to the issue reported here)
b) If not, figure out how else CTools plugins cache can be cleared and document it.
c) If CTools plugins cache cannot be cleared, open issue on CTools and remove cache = TRUE from the hook above until this issue is resolved upstream.

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1) I copied simplepie.inc into feeds/libraries.
2) It did not show up as a selectable parser.
3) I went to admin/settings/performance and hit the "clear all caches" button.
4) It was then selectable.

If one or two more people reproduce this, then we can set to "Needs documentation" and just make a quick documentation fix.

Needs documentation, yes.

I've added a note on clearing caches to the FAQ's

http://drupal.org/node/836618#comment-3217270