Maya installations with Bifrost Extension include a compatible version of Arnold for Maya (MtoA). However, if you have installed a more recent version of Bifrost, then you might require a more recent version of MtoA as well. For the minimum required version of MtoA, see the release notes for your version of Bifrost — alternatively, simply use the latest version available.
To download the latest version, visit https://www.arnoldrenderer.com/arnold/download/.
For help on installing Arnold in Maya, see https://docs.arnoldrenderer.com/display/A5AFMUG/Installation.
For renderers that do not support rendering Bifrost content directly, you can export to standard formats such as Alembic and OpenVDB.