To make Arnold your default renderer you must execute the following steps:
Create a new empty scene in CINEMA 4D.
Go to the render settings and select Arnold Renderer.
- Define your default settings (e.g. resolution, samples, etc.).
Save your scene to the C4D install folder (where the CINEMA 4D executable is located [c:\Program Files\MAXON\CINEMA 4D R18 on Windows, /Applications/MAXON/CINEMA 4D R18 on OSX]) using the name new.c4d.
- Now when you open a new scene, it should contain your settings by default.
Note that using a default scene as described above stores all render settings. This means that if a default value changes in a new release you will still see the old value in a new scene. In this case, you have to remove new.c4d, create a new empty scene and save it again.