Hi vk590,
Please open a web application with Visual Studio 2015 and then please open the menu Project -> Manage NuGet Packages. Searching AjaxControlToolkit in NuGet Packages Manager -> Browse tab. Then you can install it from the manager as below image shows.

After installing, the ASP.NET AJAX CONTROL TOOLKIT.DLL will add to your project automatically.
Best Regards,
Weiwei