One of the major changes in 3ds Max 2018 is the removal of the NVIDIA mental ray rendering engine and the addition of the Arnold rendering engine. However if you install 3ds Max 2018 as a part of a suite, the Arnold rendering engine is not included. This can only be an oversight on Autodesk's part when the installer was created for the suites. The good news, however, is that you can download the Arnold renderer directly from the Solid Angle website:
Simply download the installer, run it and you should have a fully functional Arnold renderer in 3ds Max.
For users of the Collections and "point" products (3ds Max 2018 that was purchased by itself and not as a part of a suite or collection), consider downloading the installer as well, since the version that is currently available from Solid Angle is actually a bit more recent than the one that is included with the shipping version of 3ds Max 2018.