Step 1. Download the correct mental ray plug in for your operating system here.
Step 2. Open the download after it completes. It should be in your downloads folder.
Step 3. Follow the onscreen prompts until the install is complete.
*If you encounter an "Error 20" licensing message upon installing Maya2016 and MentalRay please remove that application, and then reinstall using the following link.*
Step 4. Open Maya and go to windows > settings/preferences > Plug-in Manager
This window should open. Scroll down and find Mayatomr.bundle and click both "loaded" and "Auto Load." Once that is done, click refresh and close. Mental Ray is now ready to be used!