how to set fuel in CS 757 FMC

[bin801]

question about FMC Per initial page. I did read Menu, and I fly PMDG 747 also. My understanding is, unlike 747 FMC, you need put a number in ZFW or GRW in per initial,once you did that, the other WT will be calculated and displayed automatically, am I right?
But the problem is where should I get the quantity of ZFW or GRW to put in? Pound or KG? .?  I tried different digit, but always show invalid input.
in ACE the total Wt is 72996 Lb. is that ZFW or..? how should I put it in FMC?

any can help? thanks alot
can not wait to fly