In order to get the Apps platform to work for your domain, you will need to modify the CNAME and MX records to direct your domain or sub-domains to the Google platform.
Modifying MX Records For Email Access
You will need to find out what company manages (or controls) your domain. For example, you may have purchased your domain through Network Solutions, but you may have moved control to another company (DNS).
To modify the MX records (the records that relate to email), you will need to manage the DNS information. Many companies have different names for this service from zone files to advanced DNS manager.
Here is the configuration info for Google’s Mail Servers:
Here is a sample of one particular web host’s control panel where you will change the MX record: