I am also having the same problem. I have a self-hosted blog on www.remarpro.com, signed up for an account with wordpress.com and received an API key. However, how do you connect the two? If I want to use the API key on YouTube, how will it know that the totally new, separate wordpress.com account will relate to my self-hosted blog? Many of the solutions offered relate to having the akismet plugin – evidently, you could connect the 2 accounts by entering you api number with the plugin, however, it seems the question that isn’t answered, is how you connect the two accounts in general.