type or namespace could not be found

I am using ajaxfileuplod control.
I normally download AjaxcontrolToolkit and installed properly in my sysem.
and then i have following the fileuploading code.
http://stephenwalther.com/archive/2012/05/01/ajax-control-toolkit-may-2012-release.aspx
protected void ajaxUpload1_OnUploadComplete(object sender, AjaxControlToolkit.AjaxFileUploadEventArgs e)
{
// Generate file path
string filePath = "~/Images/" + e.FileName;
// Save upload file to the file system
ajaxUpload1.SaveAs(MapPath(filePath));
}
I am getting the following error:
type or namespace ' AjaxFileUploadEventArgs ' could not be found



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