I don't know if you looked at the help yet, but search for the topic "preform permeability".
I suspect you may be using Dual Domain or 3D right now, but the SRIM process (Structural Reactive Injection Molding), where you insert the fiber mat is only supported in Midplane at the moment. If you have a 3D or Dual Domain model and specified RTM as the process, you will not find the fiber mat option,. The element type that allows you to specify the fiber mat properties is the "Preform surface (Midplane)" element property.
I hope this helps.
Product Manager for Autodesk Simulation Moldflow products