All materials used in our presets are stored by default at:C:\Program Files (x86)\Itoo Software\Forest Pack Pro\matlib
In case of mental ray materials, the proper library files is the 3d_mental.mat
Please feel free to modify these materials (material libraries) to suite best your needs. Just take in mind, all these presets / material libraries will be overwritten when upgrading. Therefore I would suggest you to create your own "personalized copy" of bundled libraries and do those material changes on there.
Let me mention, that we recently published a "Creating Libraries
" tutorial covering this topic in detail. You might find it interesting.