Now that Visual Studio 2012 is available, the TFSPreview service information pages changed a little bit. In the past they had a great set of pointers for Visual Studio 2010 users what the pre-requisites are in order to connect to TFSPreview.com with Visual Studio 2010. Now it is fully focused on Visual Studio 2012 and it can be quite a journey to find what the pre-requisites are in able to connect with Visual Studio 2010.
So in order to connect to TFSpreview.com with Visual Studio 2010, you need to run SP1 and install the compatibility GDR. This GDR adds the capability to authenticate Visual Studio with the windows account (previously named LiveID) you used to create and administer TFSPreview.
You can see the full blog post here if you try to connect form other environments as well, like Eclipse or Microsoft Test Professional 2010. http://blogs.msdn.com/b/visualstudioalm/archive/2011/09/14/tfs-preview-downloads.aspx