OK, I thought that this was clear, but apparently it is not: a Blogger username and password is required to use this service at this point. You need a blog to apply skins to anyway, and I figure that if you're developing them, you should have an account to test them on as well. You cannot just make any random username/password combination work. This may change in the future, but as of right now a Blogger username and password is required to use this service. Thank you.