Having installed both windows 8 Release Preview X 64 and Windows Server 2012 I noticed that there was an extra update installed with Windows 8 Release Preview x64 once you installed it and you did not get this update with Windows Server 2012 RC after installing it and yet the flash player on Windows 8 Release Preview x64 works find.
So to fix the problem with the flash player on Windows Server 2012 RC all you need to do is to locate the feature User Interfaces and Infrastructure and click on it to expand it there you will see the feature Desktop Experience that you need to install. Once this is installed you need to re start the computer and check for updates and there should be an update to install.
Once this is done all you need to do is to adjust the security setting accordingly in internet explorer and it should work fine.