What is the relation between the hydrogen used and the output of the fuel cell?  For instance, I have x amount of hydrogen, at what capacity will the FC work, disregarding the Load?

The fuel curve inputs tell you how much fuel the generator consumes to produce electricity.  The sample file to which you refer has a fuel curve intercept coefficient of 0 and a slope of 0.06 kg/hr/kW.  So to produce X kW, it will consume 0.06X kg/hr of hydrogen.