Installing TinyMCE plugins on Drupal 7 with the WYSIWYG module
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()