Here are a few guidelines to review that may help in the setup:
Choose the setup option to the left hand side of the screen, next choose domain. Be sure to enter in your desired CName in the Custom Domain section, this cannot be "www". Rather is it should be something such as "signup.yourdomain.com or launch.yourdomain.com."
Once you have entered this information you will need to update your DNS setting to redirect your main URL to the page. Please see the following instructions on how to setup your A Record:
Please login to your domain host and add four new A Records IP addresses to redirect your page. Here are the IP's you need: