A few weeks ago I posted about an issue that arises when 3ds Max 2018 is installed as a part of the Building Design Suite. The Arnold Renderer is not included in that install. It's completely missing from the Building Design Suite - the workaround is to install the Arnold Renderer separately - read the blog post here if you need it.
Another omission has surfaced with the Building Design Suite - 3ds Max Interoperability for Revit, the component that allows you to link a Revit project into 3ds Max, is also missing. That means that if you installed 3ds Max 2018 from the Building Design Suite, not only will you not be able to use the Arnold Renderer, you won't even be able to link your Revit project into your scene!
Again, the workaround is to install the interoperability component separately, by following the instructions in this knowledgebase article.
Note - only 3ds Max installations done from the Building Design Suite will exhibit these two omissions - if you purchased the AEC Collection instead, you should have no missing components with your installation.