You don’t see the Plugins menu because 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 have tons of great plugins pre-installed, so chances are, we have a feature that accomplishes what you’d like to do with a plugin. What would you like to achieve?