This question comes up often. Most folks don't understand that there is some "magic" between the server where their web site "lives" and the domain name [www.yourdomain.org] of their website. Using a similar technique, you can also have a number of different domain names that all point to the same site.
Under a paid plan, the Site Creator will allow you to post domain names for your site. It's very easy.
The part that can be tricky is getting the domain registrar to change the settings for you, or just doing it yourself.
The short answer: The IP address you want to set your A record to is IP 72.20.110.40
The longer, more complete, step-by-step version:
Step 1: File your domain name(s) under My Domain in Site Properties
Step 2. Set your domain to point to our IP address
The approach to this step depends on the level of control you have over your domain name. For each option please allow up to 48 hours for the change to get fully propagated.
Option A. You can manage the DNS for your domain from the registrar's website
Login to your registrar's website with the username and password for your domain management account.
Locate the option for DNS Management (different registrars may use different names for it, e.g. DNS Control). Set the IP (A record) of your domain to IP 72.20.110.40.
Create an alias (CNAME record) "www" pointing the record you created.
If your domain is registered with GoDaddy.com, please follow these step-by-step instructions.
Option B. You (or your hosting company) manage the DNS servers for your domain
You need to set (or ask your hosting company to do so) the IP address of your domain to 72.10.110.40. It is recommended to also create an alias (CNAME) for www.yourdomain.tld pointing to the same IP address. Here you can see an example of a DNS zone file configured for us.
Option C. You can't manage your DNS from the registrar's website
You can use one of the many free DNS hosting services like http://www.xname.org, http://www.everydns.net, http://www.zoneedit.com. The interface in each one of them is different, but your goal is the same: to create "A" record pointing to IP 72.20.110.40 and add www alias "CNAME" pointing to this record. Here is an example how to do this with EveryDNS.