if you picked signup.yoursite.com as your address, enter signup, and then enter host.launchrock.com.
Congratulations! Your CNAME record is now configured to point to LaunchRock.
If you have trouble configuring your CNAME records, you can check out the MyDomain tutorial page Or, view a sample message to use when you contact MyDomain for assistance.
Once you've modified your A records, it may take up to 72 hours for changes to update live.