For this example, I'm going to use "accountdomain.com" to represent your primary account domain name and "newdomain.com" as the new domain you are wanting to park or addon.

Parked Domains - Parking a domain just points the new domain name to your primary account domain. So, if you were to park "newdomain.com", it would point to the same directory that "accountdomain.com" is pointing to. 

Addon Domain - This is usually the option most are after. Adding a domain on makes it appear as if the new domain is hosted on it's own account, even though the files are still located under your primary domain. If you were to addon "newdomain.com", a user browsing to your site will see "newdomain.com" in the URL, but it will be redirecting it to a specific folder under your primary domain.

For example:

www.accountdomain.com/new would be the folder I setup for www.newdomain.com. Whenever a user browses to www.newdomain.com, they will only be able to see the files within the www/new/ folder on your account.

How to setup an addon domain:

If you registered your domain with someone besides iServerGo, make sure that the name servers are pointing to ns1.westservers.net and ns2.westservers.net and wait a full 24 hours before proceeding with the rest of this guide.

Login to your iServerGo cPanel with the URL www.accountdomain.com/cpanel [putting in your domain name, of course].

Click on the "Addon Domains" menu option under "Domains and Stats" >> Complete the form as follows:
        New Domain Name: newdomain.com
        Username/directory/subdomain name: new
        Password: apassword

Whatever you put in the "Username/directory/subdomain" name will be the folder that is created under your primary account. It will also be the default username for mail and FTP.

If the propagation from the first step is complete, typing in www.newdomain.com should then work.