As of November 2011, new WordPress.com accounts are no longer automatically assigned an API Key. API Keys are currently used only for activating the Akismet service for blogs and sites that are not hosted on WordPress.com.
WordPress.com is gradually phasing out use of the API Key.
How Can I Get an API Key?
If you registered your WordPress.com account before November 2011, an API key was included in your WordPress.com Welcome Email. You can find it in that email if you have saved and/or printed it.
If you need an API Key for activating the Akismet service on your site or blog, please visit the Akismet site to select a plan and receive your API Key.
What About the WordPress.com Stats Plugin for Self-Hosted WordPress?
The WordPress.com Stats plugin is no longer actively maintained and you should not use it on new WordPress installations or for WordPress sites that are using WordPress 3.3 or later.
WordPress.com Stats are now integrated into the Jetpack plugin, and we recommend the use of Jetpack for new WordPress sites.