The Operator Graph (Arnold Menu) window is only available from 3ds Max 2019 onwards.
Operators Graph Editor window (Arnold menu & Render Setup)
Operators should work seamlessly in the Active Shade window.
Tutorials about Operators can be found here.
Operators branches can also be saved to .ass file with RMB + Export This/All, and re-imported (or imported into another Arnold plugin) with RMB + Import Operators.
All the nodes at the scene level root are simply prefixed by '/'. So for instance, if in 3ds Max "sphere" is at the root level, it is exported to Arnold with the name "/sphere". Nodes under a hierarchy are exported with a '/' separator for each level, starting from the root. So for instance, if in 3ds Max "sphere" is at the root level, and "cube" is under "sphere", then "cube" is exported to Arnold with the name "/sphere/cube".