For datetime fields that have datetime weblet as their the default visualization, you usually do not need to manually add it to your web page. Simply include your datetime fields in your web_map or in a list that is present in your web_map and they will be visualized using the datetime weblet. Similarly fields of type date and of type time will be visualized using the date (std_date) and time (std_time) weblets.
If you do need to add the datetime weblet to your page manually, simply drag the datetime field from the Fields tab onto your page. Alternatively, open the XSL for your webroutine in the LANSA Editor and follow these steps: