[Plugin: WP Page Numbers] link to css files

Not everyone uses the standard wp-content folder for their plugins.

WP_CONTENT_DIR and the corresponding WP_CONTENT_URL for the entire wp-content directory

These also exist, but you probably don’t need to use them:
WP_PLUGIN_DIR — plugins directory (full server path)
PLUGINDIR — (deprecated) plugin directory relative to ABSPATH

I recommend putting this in the start of your plugin somewhere:

if ( ! defined( ‘WP_CONTENT_URL’ ) )
define( ‘WP_CONTENT_URL’, get_option(‘url’) . ‘/wp-content’ );
if ( ! defined( ‘WP_PLUGIN_URL’ ) )
define( ‘WP_PLUGIN_URL’, WP_CONTENT_URL. ‘/plugins’ )

…and then use URLs based on WP_PLUGIN_URL when calling scripts

http://wordpress.org/extend/plugins/wp-page-numbers/