Okay, here's what I was able to find in the manual, but when I get a chance, I'll look at my FMC and see if I can find anything else.

Try these steps... I don't know if they'll do anything, but it's worth a try:

1. Click "Perf Init"
2. click Button 2 (left hand side of buttons, second from the top. It should be next to "Fuel" on the screen--basically, just click "Fuel")
3. See if you can enter the amount of remaining fuel (Aircraft>Fuel and Payload)

Post what happens.  ;)