First you need to add the reference of AjaxControlToolkit dll
Follow the steps to add the reference :Right Click on Project in Solution Explorer.
Click on Add Reference in cascading menu.
Click on Browse tab
Locate your dll.
Click on OK.
Now register the assembly:
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp"%>
Then try dragging and dropping the controls.
NOTE: The dll versions should be same which you added in toolbox and referenced in solution explorer.