Stolen Mac Calls Home - Report your Mac Stolen
In your "Mac Calls Home" Account page select appropriate Product ID and you will see screen similar to the one below.
Just check "Report this Mac Stolen" checkbox and "Update Information". "Mac Calls Home" will do that for you automatically when theft report goes through.
In case you made a mistake during login on your Mac and it is NOT stolen - just uncheck the checkbox and update information.