The solution is actually rather simple. I host my own wordpress blog and was having the same problem, but here’s what you need to do:
1. If you haven’t already signed up for a wordpress username, do that at wordpress.com [Note- when you sign up, there is an option to just sign up for a username, without a blog. I don’t know if that’s necessary, but that’s what I did.] For those who have mentioned that you can’t get a new username because you’ve already used your email address once, get a new email. You can sign up for a new email in a matter of moments at so many different services (Gmail, yahoo, etc.)
2. Log in to your wordpress username account and your API should be in clear view as some have already mentioned. Copy this key onto your clipboard.
3. Log-in to your self-hosted wordpress blog. Go to your plugins and try to activate one of them. When it asks for your API key, enter the one you have copied onto your clipboard. If it works the way mine did, a prompt will tell you that the api key matches a different username (it should be the username attached to the api key) and it will ask if you want to add this blog to that username. You do!!
4. Celebrate because you are now linked up to a wordpress profile with an API Key!
Good luck! ??