If you don't program the FMC, LNAV and VNAV are unable to be used. You need to use the A/P like a normal one:

SPEED HOLD (IAS or MACH), ALT HOLD, VS (for the climb and descent phase), HDG Hold or LOC for tracking VOR. Program the FMC and it does most of the rest for you.

The FMC and the Autopilot are a team that work well together for all stages of the flight. At the Top of Descent (TD or TOD), the aircraft will descend at a nice rate and it will (or at least should) be at the correct altitude at the time you need to press LOC to capture the ILS localizer and then APP to capture and track the localizer AND Glide Slope. Also. By using the FMC you can do an AUTOLAND which upon landing, cuts the throttle and flares the aircraft so it gently touches down. Without the FMC, you will need to turn off the Auto Throttle or the aircraft will not let you put the reversers on to slow down. I have done enough flights where the aircraft has refused to stop and races of the end of the runway, into the bushes while I try to figure out why my reversers are not working! :-[

Because of the similarity between the 757 and 767, I recommend you do the FMC tutorial (for the 757) using the 767 instead. You can find it here: http://www.captainsim.org/yabb2/YaBB.pl?num=1220997876. It will open a whole new way of flying for you.

I  only started learning to fly the "Heavies" in early 2009 and it opened a whole new experience to flying that I never knew existed. I'm just mad at myself that it took me years of flight simming (I started in late 1983) to start flying them. Admittedly the early versions of Flight Simulator, were GA only, but when they first showed up, I was only interested in GA flying and wouldn't touch them.

I still love flying GA, I own a whole bunch of GA add-ons for FSX, but I also love flying the Heavies now and I fly them the most.

Mark