For Arnold 5
This manual introduces HtoA, a plug-in for Houdini which allows you to use the Arnold renderer directly in Houdini. Before beginning, you should read the sections on installation and licensing. There is also a short introductory tutorial taking new users through the steps necessary to get their first Arnold render using HtoA. Also, don't forget to look at the Tutorials section.
Make sure to check the release notes for the latest Arnold 5 features. There is also an FAQ page that endeavors to answer any questions related to transitioning to Arnold 5. Ensure that you are using the latest version of the plugin.
Arnold Answers is a reputation-based forum where Arnold users of all kinds can ask their colleagues and peers questions related to Arnold.