I'm working on a power law flow through a porous media, using Fluent simulation. I know that the user guide "Using the Power Law Model" statement is incorrect and I don't know how to resolve this problem (the n and m parameters in viscosity power law material panel overwrite C0 and C1 in the fluid panel) Thanks for any help!

This is the typical question you must ask fluent support. They really have specialists in porous media and with fuel cell it is the strong point of fluent expert. You really should submit a question from the support website to get the best answer.