The common library of Autodesk rendering materials (Protein) for AutoCAD, Revit, Inventor, 3ds max and other CAD and multimedia applications of the 2011 family (installed with these products) is stored in the folder (fixed location):
C:\Program Files\Common Files\Autodesk Shared\Materials2011
and in 64-bit OS in the folder:
C:\Program Files (x86)\Common Files\Autodesk Shared\Materials2011
and the mapping files for texturing and bump maps then in the subfolders assetlibrary_base.fbm\Maps.
Since version 2012 you can find the "Autodesk Material Library" in the folder:
C:\Program Files\Common Files\Autodesk Shared\Materials\2012\ (etc.)