[Plugin: WordPress.com Stats] need API keys

I’m assuming from your topic title that your question may be specifically “where do I get an API key?”, but the rest of this is equally important.

“Future upgrades to WordPress.com Stats will only be available in Jetpack. Jetpack connects your blog to the WordPress.com cloud, enabling awesome features“

“Installing this stats plugin is much like installing Akismet, all you need is to put in your API Key“