http://faq.wordpress.com/2005/10/19/api-key/
You need to register at http://wordpress.com for an account there and recieve the API in the welcome email or under http://dashboard.wordpress.com and then “profile”.
Trent
I had a friend help set up my pluggins and wordpress.org for me, now she cannot access any of her pluggins. It tells her that her api key is wrong and that she is somehow connected to my blog and should use my api key. It will not let her use her api key. All of our stats are mixed up together. She has removed and reloaded several times, and still the problems exist. What can we do?