If you don’t see any of the Arnold render channels (like Arnold_Alpha or Arnold_Opacity), open up the script editor (press ALT+4) and run the SITOA_CreateRenderChannels() command.

Tip – You can avoid this by making Arnold the default scene renderer. The SITOA plugin will then make sure the Arnold render channels exist (by calling SITO_CreateRenderChannels from OnNewScene and OnStartup events).
hat tip to Francois Lord for pointing this out

# Python
Application.SITOA_CreateRenderChannels()

// JScript
SITOA_CreateRenderChannels();