In Maya 2015, we usually use mental ray to render our model, some new users may not see the mental ray item, that is because you have not loaded it yet.
Go to Window -> Setting/Preferences -> Plug-in Manager
Find Mayatomr.mll, select both Loaded and Auto load, then you will find mental ray is ready to use there.
原文：http://www.cnblogs.com/grandyang/p/5406647.html