Loading user controls dynamically is one of the most common scenarios in ASP.NET programming. This example demonstrates how to achieve the desired functionality in a straightforward and simple way with the RadAjaxPanel control. You can also use the RadAjaxManager for the same purpose -- you will need to ajax-enable a standard asp:panel and then load the user controls within.

Dynamic loading of user controls with AJAX is also discussed in the following online help resource:
RadAjax help -> How-To -> Loading User Controls with AJAX

Relevant resources at the end of the topic could help a lot as well.