If you want to register new additional domain name (without new web hosting account)
Then you can do it here:
NOTE: You have to point new domain to the same DNS (Nameservers) like your main domain name.
Your DNS you can find in your web hosting Setup-Email.
Then you will be able to add it to your current hosting account (to the CPanel)
If you just want to point second domain to main domain (to same directory), then
see link "Manage Parked Domains" in section: "Domains"
If you want two different sites then click on link "Manage Addon Domains" in section: "Domains"
Then you can upload to new domain by FTP.
Login will be: email@example.com
Passw: your pasw
Also, your new domain will be accessible as folder of the main domain:
(but visitors will know only read site url: www.yournewdomain.com)
Good flash tutorial about it: