3ds Max 2018 includes the Arnold 5.0 rendering engine, along with the Advanced Ray Trace (ART) renderer, and no longer includes the legacy mental ray renderer that so many people have been using for the last several years.
I'm the sort of person who tries to adapt to new technology whenever I can, and I don't usually have much trouble leaving the past in the past. So I've spent the last few months getting up to speed on the Arnold engine, specifically for use in Revit to Max rendering workflows, and I've become much more comfortable with it than I was when 3ds Max 2018 was initially released. In fact, I'm finding the quality of the images, especially interior scenes, to be superior in many ways to what I could achieve with mental ray or ART.
However, if you want to use the Lighting Analysis Assistant for 3ds Max, which is still included, you need to be using the mental ray engine. Good news - you can still use it in 3ds Max 2018!
The mental ray rendering plug in is available as a free download from NVIDIA and can be obtained here. Note - the free license is good for a single stand-alone installation only, and does not include render-farm capabilities. Unfortunately, if you want to use mental ray with Backburner to run a render farm, you will need to purchase licenses from NVIDIA.