You have a WordPress.com blog, it’s entirely separate from WordPress.org.
They don’t allow plugins on WordPress.com for security reasons. Plugins are a feature only available to self-hosted WordPress.org blogs. You can find out more about that here: http://support.wordpress.com/plugins/
They do have several built-in features that were inspired by many popular plugins. You might find what you want by searching for it at http://en.support.wordpress.com/
For more information on the differences between WordPress.com and WordPress.org, please see: http://support.wordpress.com/com-vs-org/
Also, for help with your WordPress.com blog, contact http://en.support.wordpress.com/contact/
