Copied an older post of mine. hope it helps. BTW since PMDG is payware, this query belongs in the "payware" section, youäll find it on the very bottom of the forums page

Anyways here's the post

"Roughly, the FMCs on different aircrafts works the same way, and needs to know the same information in order to work.
But the actual interface and programming may be a tad different, this is very evident when comparing a Boeing FMC and the Airbus counterpart.
But there are some differences in the Boeing models aswell, but that's due to the different FMC manufacturer (Smith's industries for the 737's, and Honeywell/Pegasus I believe for the big Boeings)

I took the liberty of posting a reply I made in another thread regarding Boeing FMCs. It's pretty straight forward with no technical jibberish   

"The FMC is your best friend (aside from the blond flight attendant) 
It keep tracks of a huge navigation database: departure procedures and approaches, aswell as your current route. The FMC is linked with the Autopilot, flight director, autothrottle and the IRS. They inturn form the FMS (flight management system) 
 
However the FMC also knows the capabilities and LIMITATIONS of your aircraft, so it can plan your flight and give you the most economic values. To accomplish this, you have to feed the FMC with some  parameters. 
 
What the FMC want to know is this (chronologic order, corresponding FMC buttons in bracketts) 
 
- Current position (MENU/FMC/POS REF) 
- Route, origin and destination airport (RTE) 
-Take off runway and the SID (DEP ARR) 
- Routing; waypoints enroute (RTE or LEGS) 
- TOGW, ZFW, Fuel, fuel reserves, cruise altitude etc (MENU/FMC/PERF INIT) 
- Thrust derate (N1 limit) 
- Takeoff performance; Flaps, v-speeds, cg and trim (MENU/FMC/TAKE OFF) 
 
Now this is done step by step so when you have executed one step, you'll automaticly be presented with an option to jump directly to the next step by clicking LSK 6R (Line select key 6 right), but I posted their complete path if you get lost on the way"

Enter all the information you can. If you don't know, put random numbers.

for the route, put a ICAO code, such as YSSY into the departure and KLAX into the arival. Only change the ICAO codes to your airport.

If you weight 500 000pounds, enter your weight as 500.0. Put cost index 50-10 000.

Takes some getting used to. I have the PMDG 737 then I got the LAGO Md-80 and PMDG 747. Both have similar FMCs.

Also, reading the manual did it for me, or you could try an online one which might be here at simviation or at www.avsim.com

While we're on the subject of the PMDG 747, does anyone know how to get the vortices from the wintips to show? Does it depend on atmospheric conditions????

The Airbus A340 and Boeing 744 have quite different interfaces to their respective "FMC".

The PMDG 744 actually comes with a VERY detailed 60page FMC guide which guides you step by step.

I'm not trying to start anything mate, but look at it from his side: I dont think he wants to be told "well lad, go ahead and fly the DC3" because it wont really help with his problem. Now he probably wont take it that way, but you never know, weirder things have happened around here 

That was my point, no need to feel attacked 

Having just read the full 49-page PMDG 737 FMC manual, I feel that that would be the best bet.  As Nexus said, it seems like the 747 and 737 FMC are fairly similar.  

Before you start anywhere on this FMC thing though, make sure you have the proper software and stuff.  Make sure you have the correct AIRAC navdata, and the correct SID/STAR files.  

If you've taken care of those though, and still are confused, report what specifically what you're having trouble with, and we might be able to help more.

Otherwise, read the 747 FMC manual that the package came with.