Why is not this added

I use VS 2010 Professional

I save my web project by selecting File ->save all
Then I have download the AjaxControlToolkit.
Then I use Nuget to install AjaxControlToolKit. When I do this the bin directory is created in the solution  it looks like this

Bin
   ar
   cs
   de
   es

and so on

Now as the third step I add a tab in the Toolbox and select the AjacControlToolkit.dll so about 40 contols is added to the new tab named AjaxControlToolkit.
If I now for example drag the extender control CalendarExtender from the toolbox to a new aspx page the
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
is now added so when I run the page I get this error
Could not findany resourcesfor the specifiedcultureorthe neutralkultureCheck that AjaxControlToolkit.Properties.Resources.NET4.resourcesembedded inorlinkedto the compositionAjaxControlToolkitcorrect at compilation time,or that all necessarysatelliteassemblies canbe loadedand isfullysigned.

So what is it that I have missed when this
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
is not added

//Tony