Hi there!
We don't allow plugins on WordPress.com for security reasons. Plugins are a feature only available for self-hosted blogs provided through WordPress.org. You can find out more about this topic here: http://support.wordpress.com/plugins/
However, we do have several built-in features that were inspired by many popular plugins. For the Pinterest button, that's included as part of the Sharing feature. Here's more information on Sharing and how to set it up for your site:
http://en.support.wordpress.com/sharing/
I hope that helps!