You'll first need to login to your downtownhost.com account, or the welcome email and you'll hopefully found some nameservers. They'll look something like "ns54.downtownhost.com", there should be two. Now login to your account at yahoo and select the DNS section, there should be an option to set the namservers, add the ones from the above step here. Now you'll have to wait, it depends upon you ISP and yahoo how long it takes, but it should be quite fast. Once this step is completed your domain will point to the downtownhost.com server, once that is done you can Login.
For the sake of including the information, you login to cpanel at http://YOURDOMAINHERE.COM:2082 with the information provided in your welcome email (or downtownhost.com account section).