Hi Ramkumar,
The error message returned in in Firebug,
> CORS origin request blocked: The same origin policy disallows the remote resource at https://<portalserver>/arcgis/sharing/rest/oauth2/registerApp (reason: CORS header 'Access-Control-Allow-Origin' does not match 'http://<portalserver>, *'
This part "'http://<portalserver>, *'" is odd. Portal for ArcGIS allows CORS requests by default, but does NOT use wildcards. The fact that the ‘Access-Control-Allow-Origin’ header includes a ‘, *’ at the end likely means it is being inserted by the web server where the Web Adaptor is installed. This is likely causing a mismatch and the web browser is blocking it.
The reason a CORS request is being made in the first place is because the Portal My Content page is being accessed over http. During the app registration process, the request to the oauth2/registerApp endpoint is always done over https. The easiest fix would be to simply access the My Content page over https. Or you could investigate the web server where the Web Adaptor is installed and try and determine what is adding the wildcard.
Hope this helps,