Please follow the steps mentioned in the kb: http://helpx.adobe.com/creative-cloud/kb/aam-lists-removed-apps-date.html and try installing again.
You may try the direct download in case of any issue: http://prodesigntools.com/lightroom-5-ddl-comparison-vs-lr4.html .
Kindly follow the very important instruction before download.
Thank you for the very comprehensive response. All fixed.