I contacted MakerFarm about my problem and Colin advised me that I had placed the endstop connector plugs onto the RAMPS board incorrectly. He directed me the the connection schematic in the build instructions, and after having studied them, I was able to hook them up and get the machine working.

He also directed me to an addition to the Troubleshooting section which shows how to check that the wiring of the endstops is correct.

Old Man Emu