Where to put the serial key? Batch rendering with Arnold watermarks?

[Stephen.Blair]

You don't need to put the serial key anywhere.

When you install MtoA, the installer registers the serial key and product id.

[walid.yazbeck]

I have the Arnold 2018 for Mac browser download and when it's expanded into "Arnold_SDK_2018_5.1.0.0-mac" folder with doc, license, materials etc... where should I place this folder?

[Stephen.Blair]

You can download MtoA from solidangle.com

If you want to batch render with Maya, you need MtoA.

The Arnold_SDK download is just Arnold. It does not include a plugin for Maya, or for any other software.