WordPress.com does not accept domain transfers. What is possible is purchasing an annually renewable $13. per year domain mapping upgrade to map to an existing domain.
In order to map to a domain one must have an underlying .wordpress.com subdomain blog to map from. If you have registered a .wordpress.com blog then you log in as Admin under the exact same username account that registered the blog and purchase an annually renewable domain mapping upgrade for the blog.
See here for mapping an existing domain that you already own http://en.support.wordpress.com/domains/map-existing-domain/
Dashboard > Store > My Upgrades is where you do the primary blog set up knowing it can take between 24 - 72 hours for domain name propagation to take place throughout the internet. You can view the DNS changes here > http://www.whatsmydns.net/ What's important during that stage is to be patient.