Kyle Jones created a very helpful video tutorial explaining how to use KB Gradebook and demonstrating its features. Check it out.
I haven’t updated this plugin since releasing it some time ago. The reason is simple: I no longer use WordPress for course websites. Instead, I use a custom syllabus editor that I wrote, with grade viewing (basically the same code as this plugin) integrated. But although wp.org tells you that this plugin is only tested through WP 2.5, I hear that it still works just fine in more recent versions of WordPress.