Turn on Arnold Render Windows > Settings/Preferences > Plug-in Manager
Scroll down to the bottom and then check both boxes next to mtoa.mll
Right-click on the object and choose Assign New Material. Select the Arnold render material options in the drop-down and choose aiStandard.
Right-click on the material you want to change and choose Material Attributes.
Here is a link to more materials: https://support.solidangle.com/display/ARNTUT/Standard+Material+Presets