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.

本文转自博客园Grandyang的博客，原文链接：Load Mental Ray in Maya 2015，如需转载请自行联系原博主。