Parked Domains are additional domains on your account that go to the same place as your main domain.
 

The first step would be to e-mail support requesting a parked domain.
After you receive the details you would then log into your cpanel and click "Manage Parked Domains"



Next you would fill out this form:



Make sure to replace ParkedDomain.com with the actual domain you are adding.


After this press the add button. After this the domain should be added.
You will also need to change the nameservers on all parked domains.
Please e-mail support if you need the nameservers.