How can insert fuel cells, for example, PEM, to cover the electrical needs (an electrical load)?

A fuel cell is a type of generator, so you use the generator module to model a fuel cell in HOMER.  Please refer to the FAQ page of the HOMER website for more information.

If the system will electrolyze excess electricity to make hydrogen that the fuel cell will then consume, then you should add an electrolyzer and a hydrogen tank to the schematic using the Add/Remove button.

If the fuel cell will consume an externally-supplied fuel that goes through a reformer before entering the fuel cell, then you have to use the generator module to model both the reformer and the fuel cell.  That is, your cost inputs should include both fuel cell and reformer costs, and the fuel curve should show the electrical output of the fuel cell versus the fuel input to the reformer.  You should not add a reformer to the schematic unless you intend to serve an external hydrogen load.