You will receive this message if you are using a developer release of Mac OS X v10.6 Snow Leopard. You must install the public release of Snow Leopard to resolve the issue.
To check which build of Mac OS X v10.6 you are using, click the Apple () menu, and choose About This Mac. Then, click the 10.6 version number one time to display the Mac OS build number.
On Mac OS X v10.6, the build number will consist of both letters and numbers. The build will start with "10A", followed by a 3 digit number. If the 3 digit number is lower than 432, then you are using a developer or Beta (pre-release) version of Mac OS X v10.6.