Help with Ifly 747

I download the Ifly 747 the other day as I mainly fly the 767 but was looking for a freeware 747, but I am having a few problems setting up the 747 for flight.

So to test this aircraft I decided to try a short flight from Klax-Ksea. I use Fsbuild to plan my flights and get my fuel requirements. I found a 747 with a similar zfw so I used that to calculate the fuel which gave me a total of around 57,000 lbs I loaded up fsx and entered my fuel into the fuel/payload screen. I decided to load extra fuel and decided to load 65,900 evenly between the left and right tanks. When I went into the flight though the fmc kept saying insufficent fuel.

In what order should I load the fuel tanks if I am not using very much fuel?

Also when I try and manually enter a zfw or gross weight then I get an invalid entry error. Like I cant manually change the fields even though I converted the weights to kgs Any ideas?

Thanks