A little google will help out. StructuringElementEx doesn't exist anymore in the new version.
In the new version, use something like this:
Mat rect_6 = CvInvoke.GetStructuringElement(Emgu.CV.CvEnum.ElementShape.Rectangle, new Size(6, 6), new Point(3, 3));
Check this link for more info:
http://www.emgu.com/forum/viewtopic.php?t=4946[
^]
Good luck!