Actually it is possible that the widget he is referencing is a layout widget and not supported by the Flex Application Builder.
now I would like to include the above widget into ArcGIS Viewer for Flex Application Builder 3.1.. If he has developed a widget that has to be added as a UI Element widget than it is not going to work in App Builder. But yes as you stated he can manually add the widget using the steps you provided. I believe that because the op stated he was attempting to ready his widget for App Builder that he was responding to Sarthaks responce when he said
I have followed the above steps but no luck. could you please suggest me another alternate.
Uday,
Just wanted to confirm if you have looked at: Managing custom widgets
From the previous link I mentioned-"Starting with version 3.0, it is possible to add and remove custom widgets from application builder. A dialog box is provided to make adding and removing custom widgets as simple as the click of a button. For more information on adding custom widgets into application builder, see Manage custom widgets."