You have bought exactly the right part.

1) There is a portion of the old plastic piece still in the regulator. It is the steel piece in the center of the plastic. You must remove this piece. To remove it, get a Dremel tool and grind the raised portion of the steel pin on the back side of the regulator. It is peened over and you must grind off all the peened metal to get it out.

2) Now that you have a hole in the regulator, you can insert the pin from the new plastic piece. BUT DON'T DO IT NOW. Carefully take the plastic piece apart. It is a three piece deal. Two plastic halves that trap a steel center.
Once the plastic piece is apart, set one half of the plastic piece, together with the steel pin, into the regulator.

3) Carefully set the regulator down so that the steel pin is pointing upwards through the regulator and the face (bottom) of the pin is supported on a very rigid surface (steel table). BE SURE that the steel is supported and NOT the plastic.

4) Now, carefully peen the steel pin over with a ball-peen hammer to mushroom its face down onto the regulator. It's quite soft so this is not difficult, PROVIDED that you have properly supported the bottom face.

5) Reassemble the plastic pieces together (don't lose the rubber bumpers inside) around the steel central portion.

6) Put the regulator back in the vehicle.

Good luck.