I'm not with Laminar Research, just a fellow simmer in the UK and I fly the Zibo 737-800X and 737-900U quite a lot.
As you probably already know, when you enter a flight plan in the FMC, an altitude profile will also be set up along the following lines:-
1. Departure and ascent to TOC
2. TOC to TOD
3. TOD and descent to arrival
Before departing, the autopilot altitude display located in the centre upper console has to be set to the maximum flight level shown in your flight plan i.e. the top of climb altitude, say FL 36,000ft.
Importantly though when trying to descend again and before you get to the TOD point, you have to remember to reduce the autopilot altitude by using the rotary control switch in the upper centre console, otherwise the autopilot will simply stay at your maximum flight level.
When flying the aircraft in the real world, ATC would probably vector you down to given flight levels as part of your descent profile and you would then alter the autopilot altitude to the appropriate level each time.
However when flying a simulator without ATC control, all I do is reduce the altitude by using the rotary control switch (before TOD), to the glide slope platform altitude for the destination airport i.e. 2000ft. By so doing, you don't have to worry about adjusting altitude again, as the autopilot will descend the aircraft for you, based on the flight plan altitude profile set in the FMC.
When the aircraft reaches and locks onto the ILS and glide slope, the autopilot will again automatically reduce altitude as required, until you switch off the autopilot and auto throttle for a last minute flare/retard for landing.
I'm currently on vacation so I'm writing this from memory, but I hope it's of some use and even more importantly, I hope that I have understood your question after having writing all of the above!!