The Calendar Control in Web Report Studio 4.2 is one of my favorite things! (See blog post at:
http://sas-bi.blogspot.com/2009/09/favorite-things-about-web-report-studio.html)
In order to utilize this, you must create a prompt in the SAS Information Map with the prompt type of 'Date'. If you don't see this option, then the data element/variable you are trying to build the prompt on is not the correct format. It must be a DATE or DATETIME valid format for the date prompt to be created. And the calendar control will only be viewable for DATE type prompts.
Hope this helps!
~ Angela
http://sas-bi.blogspot.com