The steps are:
- Create the Cryptomatte material and assign it to an arnoldAov shader slot.
- Set the Cryptomatte material on an ArnoldGlobalSettings node, in Channel Definitions -> aov_shaders (add an entry and assign your Cryptomatte material).
- Create ArnoldOutputChannelDefine and RenderOutputDefine nodes for each of the Cryptomatte channels you care about: probably some of crypto_object, crypto_material and crypto_asset.
- Make sure to leave the type set to RGBA.
- Don't forget to set the channel on RenderOutputDefine to your channel created in ArnoldOutputChannelDefine.