No air for defrost or feet just the front vents

You guys have been a blessing to me in the past. I have no air for defrost or feet just the front vents. I changed out the actuator under the dash did not help. then replaced the manual controls did not help. so assumed it was the actuator under the dash on the drivers side. My son removed the old unit but hurt his back and could not install the new one. My question is, how to install the new one? There are 2 grooves so I assume a pin goes into those grooves when installing it. Again thank you so very much for your help.