Yeah, I knew I wasn't putting all the information into the FMC, but now that I've read the FMC tutorial by aussie in its entirety, I think I know what to put it.
I had to do that tutorial flight quite a number of times before I got it sorted out in my head. Be patient and keep at it. Once you get it figured out, it's really easy.

So if you put in the Arr/Dept runways, then ATC will direct you to those runways and it will appear in your flight plan?
Not necessarily. The ATC will, IF the arrival runway you have selected is the one in use when you get there. I hate using the ATC when I'm using the FMC controlled aircraft because the ATC does NOT know which runway I have selected and with ATIS only having a range of about 60nm, it makes it difficult to adjust your arrival selection and can put a lot of pressure on you to get it done as quickly as possible. At least the ATC will get descending in time, even if you FMC does not have a Top of Descent showing. The ATC in FSX does not use SIDs, STARs or TRANs and it's sense of guidance if laughable.

And by the way, you do you figure out the Center of Gravity?
With great difficulty! I use a figure of about 30-31 and it works well. I really don't know the correct formula, but aussie has a neat way of doing it in his tutorial, even if it isn't the correct way to do it.

Mark