I think you're confused, preguntame. You're on WordPress.ORG right now (the website for the standalone version of WordPress), not WordPress.COM (the site for a hosted version of WordPress). You need to use their forums...
If you already have a domain, why not install WordPress into it and host it yourself?
Because maybe they don't have hosting?
Actually I am not sure if this is possible.
It's not. There are places out there though like Sitelutions that'll allow you to point your domain at their site where they'll then make an iframe for you. It'll show your domain name in the address bar and your website in the window.