http://www.ptc.com/products/creo/sketch/ tells that "Creo Sketch is expected to be available in late July 2011.".
I guess that the software is not ready for download ...
On page of product Creo Sketch there is a following remark: "*The timing of any product release, including any features or functionality, is subject to change at PTC’s discretion".
P.S. The link to downloading is active: http://www.ptc.com/go/sketch