To install WebMatrix Beta, use the Web Platform Installer 3.0 Beta, which you can get from http://go.microsoft.com/fwlink/?LinkID=195467

To install only ASP.NET Web Pages visit http://go.microsoft.com/fwlink/?LinkID=196189&clcid=0x409

For further instructions and information about WebMatrix and ASP.NET Web Pages visit http://learn.iis.net/page.aspx/872/webmatrix-beta-release-readme/