Installing TinyMCE plugins on Drupal 7 with the WYSIWYG module I using Drupal 7 and the WYSIWYG module and the TinyMCE (3.4.2) plugin.
I want to install a plugin for TinyMCE.

I placed the plugin folder in the right directory. 

sites/all/libraries/tinymce/jscripts/tiny_mce/plugins


However, the plugin doesn't show up in my configuration checkboxes for the WYSIWYG configuration.  What else do I need to do to enable a contributed plugin?
You need to inform the WYSIWYG module that you've installed a plugin, it won't know it's there otherwise. It will involve writing a small custom module.

Have a look in the wysiwyg.api.php and wysiwyg.api.js files included in the root of the  WYSIWYG module folder; there are examples and very good documentation explaining exactly what you need to do.

The most important (perhaps only) hook you'll need to implement is hook_wysiwyg_plugin()