The serial plug consists of three parts: an O-Ring which is held in place by a size4 allen screw and a slotted nut. To install it, you have to slightly loosen the screw and the nut. By this, the o-ring will shrink a little bit. Now, you can insert the plug into the tube of your multilink. Simply push it down to the desired position. Use the help of the screwdriver or your sze4 allen key to measure the distance within the tube. When you reached the desired position, use the slotted screwdriver and the allen key to tighten the screw and nut, hence squeezing the o-ring so that it expands and blocks the tube.
Please also see this example scetch with two serial locks in use.