If you chose signup.yoursite.com, enter signup in the Subdomain field.
In the Target address field 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 OVH tutorial page. Or, view a sample message to use when you contact OVH for assistance.
Once you've modified your A records, it may take up to 72 hours for changes to update live.