The mat file has to be loaded onto the workspace to perform operations on it.
This can be done by using the load command.
After this is done, you can access those variables the same way you would access any other variable in the workspace.
Your Code will then transalate to
for i = 1:length(s)
filename = (['MC Case (' num2str(i) ') .mat']);
demand = Demand / 365;
supply_M = Supply_M / 365;