I'm breaking Exercise 7 into 3 parts - Part 1 will cover preparing your AC Adapter, Part 2 will cover taking apart a relay, and Part 3 will cover the circuit you'll put together.
For this part, follow the instructions carefully on page 56 - cut off the end of the adapter and strip the two ends (one shorter than the other) and twist the copper strands together.
I set my adapter to 6V but feel free to set yours in the 3V-12V range. Set your multimeter for voltage and touch touch the red probe to one wire end and the black probe to the other. If the voltage reading has a negative sign (-) then reverse the probes until you see a positive voltage. Make note of which wire the red probe is touching and use red tape or a red Sharpie to mark that wire so you'll know it's the positive wire.