This article describes how to configure Hosted Email Security when using Google Apps.
Once your domain is activated you can proceed in setting up the Google Apps mail settings to work with HES.
On the Destination Server(s) field, set the email destination to point to Google Apps

Log in to the Google Apps domain management portal.
Navigate to Inbound Gateway and enter the IP Addresses of the Hosted Email Security.

Point your MX records to HES.

The MX record or Mail Exchange record is the IP address or domain name that will be receiving your mail. This has to be the first destination of your emails, which in this case will be HES. This address must be configured through your ISP or domain registrar.