well you are trying to get the DOMAIN which you DO HAVE TO PAY for and not the SUBDOMAIN which is free

so login to your bravenet accouont click websites tab and then click build a website and then the use a subdomain link and then type in a subdomain name you want

it will be xxxx.bravehost.com

and if it is available you will then be taken to the maintenance page for your site to start building with the online builders or you can use your own html editor

A domain and a web site are two totally separate things.

If you don't want to pay the registration fee for a domain name then you are at liberty to use a bravehost sub domain instead.