Variable <MENTALRAY_DIR> is supposed to point to the directory where mental ray is installed.
Make a backup copy of the file and change all instances of:
+ LOCALE:en_US mayatomr 3.13.1.8 <MENTALRAY_DIR>
To:
+ LOCALE:en_US mayatomr 3.13.1.8 D:\Installed Progs\Autodesk\mentalrayForMaya2016\
Earth: The crazy asylum of the universe.