SetDirectory[NotebookDirectory[]];
Get["EKRScaleupPower.m"];
cap20L = capacitanceParallelPlate[0.02^(2/3), 0.02^(1/3), 50]
(*1.20167*10^-10*)
You needed to change the optional argument from
freePermittivity:8.854 10^-12
to
freePermittivity_:8.854 10^-12
A call to a function can also be done by explicitly putting the context name in the call. I myself like to do that when I am using more than one package even though it is not needed, but It helps when I am looking at the code later to remind me from which package the function came from. In your case, the call would now become like this:
cap20L = EKRScaleupPower`capacitanceParallelPlate[0.02^(2/3),0.02^(1/3), 50]
(* 1.20167*10^-10 *)