System Center 2012 R2 Preview is now available for download through the TechNet Evaluation Center. This is a VERY large download so be prepared to wait a while for it to complete. This download is for pre-built virtual machines that you will need to install to a local Hyper-V Host. If you would like to install the bits directly to Windows Servers in a lab environment, see the download link at the end of the blog post.
If you would like to install the System Center 2012 R2 Bits directly to a Windows Server and configure to your test environment, this will require for you to have a TechNet or MSN subscription to download the individual installers from the TechNet site - installers.