The egr tube is the metal tube, about 3 feet long, and snakes its way behind the head with a lot of bends in it. It connects into the intake, and the blockage is typically within the first few inches of this connection. I cleaned mine with the head off, so I don't know what the easiest way would be for you to clean it. Some report going through the other end with an old flexible speedometer cable until the blockage can be punched through.
You need technical information with illustrations to guide you through this and other future projects. You should acquaint yourself with the following link:
MB Service Library