Loading MtoA isn’t enough to create the defaultArnoldRenderOptions node. The defaultArnoldRenderOptions node isn’t created until a user opens the Arnold Render Settings for the first time.

In code, you can do it like this:

from mtoa.core import createOptions
createOptions()

# Set default render options
setAttr( "defaultArnoldRenderOptions.motion_blur_enable", 1 )

When createOptions() creates the defaultArnoldRenderOptions node, it also creates the defaultArnoldDisplayDriver, defaultArnoldDriver, defaultArnoldFilter nodes and hooks them up to the defaultArnoldRenderOptions node.

Note that for setting defaults like this, you can also user a userSetup.py file. createOptions() calls hook functions that you can implement in your userSetup.py.