The SItoA plug-in is distributed as a Softimage add-on. To install it, simply download the correct SItoA pack and unzip the file. Then in Softimage, select the File->Add-on->Install menu option and point it at the SitoA add-on (.xsiaddon) file. Then follow the steps in the wizard:
So here you install all of the data (Factory, User, Workgroup). It will then install all of the files that are required to execute the Arnold Renderer.
Please refer to the Softimage documentation for other ways to install Softimage add-ons.
RUNNING SITOA on LINUX