WP_PLUGIN_DIR.'/pluginName/' will give you the absolute path to the directory of the plugin.

EDIT After comment

Plugins are plugins, theme files are theme files. Lets not confuse the two.
You cannot copy a plugin in the theme directory – because stuff does not work like that.

After careful editing you can include the plugin as part of your themes options.

Lets make an example:

1. I want to include the hello dolly plugin in my theme. So I copy hello.php into my themes directory like so: themes/mytheme/include/plugins/hello.php.

2. In order to load this file (“plugin” – but its not really a plugin anymore) I need to require_once the hello.php file like so:

   define('mytheme_inc_path', TEMPLATEPATH . '/includes/');
   define('mytheme_inc_url', get_template_directory_uri(). '/includes/');
   require_once mytheme_inc_path. 'plugins/hello.php';
   

   a. If I have a more complex “plugin” then the code will be like this:

   require_once mytheme_inc_path. 'plugins/myplugin/myplugin.php';
   // because the plugin deserves its own directory
   

   Within the myplugin.php you will enqueue the required scripts and styles like so:

   function mytheme_plugin_scripts() {
       wp_enqueue_script('tiled-gallery',
           mytheme_inc_url . 'tiled-gallery/tiled-gallery.js',
           array('jquery')
       );
   
       wp_enqueue_style('tiled-gallery',
           mytheme_inc_url . 'tiled-gallery/tiled-gallery.css',
           array(), '2012-09-21');
   }
   
   add_action('wp_enqueue_scripts', 'mytheme_plugin_scripts');
   

Caution

• Be sure to remove unwanted ‘plugin’ hooks (filter, activation/deactivation or otherwise).
• Be sure to remove all other actions except those that you really require

Conclusion

If you did some research in script/style enqueue how-to’s you’d know that plugins (that are stored in wp-content/plugins or wp-content/mu-plugins use a bit different functions to retrieve absolute/relative path and uri. Like so:

Function Reference/wp enqueue script