John, you're right; the instructions leave a lot to be desired. The connector pinout illustration on page 5 describes pin 5 of the connector as: "Intercom (ground to activate)."
Looking at the attached schematic of the external connections to the FL-760 (not the FL-760A), the only way to make the microphones "hot" for the intercom is to 1) ground pin 5 through a push-to-talk switch, or 2) ground it all the time and program the VOX (voice activation) sensitivity as described in the manual:
If grounding pin 5 and then setting the VOX sensitivity as described above doesn't work for you, call them and get some tech support.