What is the solution for "The Type or namespace 'AjaxControlToolkit' could not be found..."?

Way to reference a dll is:

1. In Solution Explorer, select the project.
2. On the Project menu, click Add Reference. The Add Reference dialog box opens.
3. Select the tab indicating the type of component you want to reference. (If you don't find your .dll in the list, then go to the browse tab and find it in your directory)

An article on MSDN to include installed libraries.

And there is another way to do it, too.

To have the needed references for AjaxControlToolkit

Right Click project/solution > Manage NuGet Packages..> Install AjaxControlToolkit.

This will add needed assemblies to your solution.