After you get logged in, you can choose to order the new domain under a new package or add it to an existing package. If you are only using 1 and 1 to register domains, I don't think it really matters which option you choose. If you have a hosting package is when this becomes a concern.
Since you are already a customer, lets look at adding a domain to an existing package.
Log In
If you login from the home page, you will arrive at your account control panel.
Select the 'Domains' option
Just below any existing domains you own, there will be an option to purchase more..
Click and enter the domain name you would like to purchase. When you click the 'Check Domain' button, a list of available domains will be returned. Again, you will be offered alternate TLD domains to purchase at the same time, but it is not a requirement.
SUCCESS!!
Follow the payment prompts and your domain will be added to your Domain Overview.
Going in and changing the DNS settings is also easy, but now it tells me I have to wait up to 24hours for the settings to take effect in WA so I am currently here twiddling my thumbs waiting for that to happen so I can start building my webpage :)