How can I use the fuel cell option?

A fuel cell is a DC generator, so if you want to model a fuel cell, add a generator and make it DC.  If you want the fuel cell to consume stored hydrogen generated by an electrolyzer, add an electrolyzer and a hydrogen tank, and specify the fuel cell's fuel as "Stored hydrogen".  If you want the fuel cell to consume reformed natural gas, specify the fuel cell's fuel as natural gas, and make sure the fuel cell inputs correspond to the fuel cell and the reformer together.  That means the capital cost should account for the cost of both the fuel cell and the reformer, and the fuel curve should indicate electricity out versus fuel into the reformer.