You can import shader networks from ASS or MaterialX files to Arnold Materials via the Utilities > Material > Import... menu or the Alt+W~I shortcut from the Material Manager.
You can not import materials from MaterialX look files (see Export MaterialX Look).
Select the ASS or MaterialX file from the file browser. The plugin reads the shaders from the file and adds new Arnold Materials to the scene.
Shaders can be imported to existing Arnold Materials from the Arnold Shader Network Editor.