The foot must be compressed and in this position when the threads of the sensor engage the pointy nut. For now just hold the two halves together while you screw it on until the second pointy nut is just threaded on, then back off a turn. Now position the top nut counterclockwise, hold the two halves so they turn together and push down on the pointy piece in order to compress the washers and thread it on. It takes a few tries. The assembly is bit too clever I think.
Repair is both costly and time consuming.
Every effort should be made to prevent an unlocked sensor from being moved, however gently. A serious blow to a locked sensor, such as dropping it, can wreck it. Since they are round, slick and have no handle you should use both hands and not be casual about carrying them. Climbing in or out of a vault is especially risky for you and the sensor, have someone hand it to you. Try not to have any green lizards jump on you from 30 feet up in the middle of the installation either.