If you can’t select ,Mental Ray as a renderer in your drop down menu under your render options ( see below ) Then it is most likely that your Mental Ray option isn’t loaded in your Plugin Manager
To solve this problem go to Window – Settings/Preferences – Plug-in Manager
Scroll down in the menu and look for Mayatomr.mll
Check both Loaded and Autoload. Mental Ray should now be loaded and you should be able to use mental Ray.
If you have any questions, please contact me at email@example.com or check out my channel at https://www.youtube.com/user/MikeHermes65