Jackie,
First, you have to verify that your account will allow add-on domains. You can check this in the cpanel. Look in the left column under general account info. It should have a line that says
Addon Domains 0/2 or something like this. Make sure it does not say 0/0, which means you can't create add-on domains with this account. You can talk to your hosting company and they will be glad to add this for a small fee
, or you can upgrade you account.
Assuming you can add at least 1 addon domain, then you need to first click on subdomains button and add a subdomain. The domain or directory where you plan to install wordpress can be called anything. It does not have to match your domain, but it might help you if you setup multiple domains to have them close.
Now, click on the addon domains icon in cpanel and enter your new domain name, for "username/subdomain name:" put in the subdomain you created above and give it a password.
Click add. That should do it.
Except for the Name server changes that I mentioned in my reply to your other thread.
Let me know if you have more problems.