Hi Ron,
Well, not sure what to tell you. If WordPress does not know where the PLUGINDIR is... then there must be more issues than this plugin not finding the stylesheets.
Have you told WordPress that your renamed the wp-content folder?
define ('WP_CONTENT_DIR','/full/path/to/your/content/dir');
define ('WP_CONTENT_URL','http://example.com/full/path/to/your/content/dirs/url');
http://wordpress.org/support/topic/renaming-moving-wp-content
It seems you also might need to:
define('WP_PLUGIN_DIR', 'new/path/to/plugins');
define('WP_PLUGIN_URL', '/url/to/plugins');
http://sumtips.com/2011/01/move-rename-wordpress-wp-content.html
Again, a Link Explaining what you did would be very useful.
Update: It seems there is a ticket open on this: http://core.trac.wordpress.org/ticket/7704
Will update plugin to use WP_PLUGIN_DIR and see if that fixes it for you.