You'll need four (4) screws and three (3) nuts to assemble this thing. It was created for use with 6-32 screws/nuts, but the .scad file should allow for easy adjusting if necessary. The screw at the bottom, the one that triggers the y-end stop, should thread into the part from the back. The screws at the top for the plunger and belt clamp should slide through the part and be engaged by the captured nuts on the back.
You'll need the shapes.scad file for the hexagon() module.
I'm still getting my Printrbot Jr fully calibrated, but this printed out really well for me. If you notice anything that needs to be adjusted, please comment below and let me know.