Visit the Arnold for Maya User Guide for comprehensive documentation on the MtoA plug-in.
Arnold for Maya is included with a default install of Maya via the mtoa.mll plug-in. This plug-in is auto-loaded, and Arnold is set as the preferred renderer in Maya.
In the case where Arnold is not loaded by default, you can manually load it via the Plug-in Manager (Windows > Settings/Preferences > Plug-in Manager) by enabling mtoa.mll.
You can choose to opt out of the Arnold installation by disabling the Arnold for Maya option during the installation process.
Pre-existing Arnold licenses are only consumed in situations where you need to render without a watermark; for example, during batch rendering. They are not consumed for interactive rendering with Maya.
Select Arnold > Utilities > Convert Shaders to Arnold from the Maya menu.
This local version of Arnold for Maya only works interactively within Maya. You can render a single frame or a sequence of frames interactively; however, batch rendering and calling from mayabatch.exe are not available and your images will appear with a watermark. See this support article Batch Rendering in Maya with Arnold Produces a Watermark.