I soldered the header onto the keypad, and used F/M jumper wires to connect it to the circuit on the breadboard.
I got the circuit to work, with one exception: The 555 timer is not putting out enough power to trip the relay, I can trip the relay manually by touching the row in the breadboard connected to the relay (I verified this with the continuity function of my meter).
The LED connected to the timer stays on for about 1 second after the correct code is entered, as described in the book. The # key resets the relay.
I see from the interwebs that others have had problems with this. One solution is to use a transistor between the output pin of the 55e and the relay (2N2222 transistor, output pin to base, collector to 150 ohm resistor and Vcc, emitter to LED and relay). This did not work for me. There is at least one video with it working without a transistor (but a different relay). Here's another with the transistor.
I'm not sure where to go with this now. More thinking.
This blog has a discussion, including the solutions I tried. Nothing seems to work for me. It may have to do with the relay. Mine works, just not in this circuit. All the videos of ones that work use a different relay.