[Plugin: WP Subtitle] Does not work?

I used the code<?getthe_subtitle($id)?> mentioned in the instructions on my singlepost.php and I am getting an error saying
“Fatal error: Call to undefined function getthe_subtitle() in /wp-content/themes/Vulcan/single.php on line 10”

Any idea how to fix this?

http://ww.wp.xz.cn/extend/plugins/wp-subtitle/