The SItoA plugin is distributed as a Softimage add-on. To install it, download the correct SitoA pack, unzip, select the File / Add-on / Install menu option, point it at the SitoA add-on (.xsiaddon) file, and follow the steps in the wizard:
Softimage will ask you where to install all the data (Factory, User, Workgroup). It will then install all files that are needed to execute the Arnold Renderer.
After the installation is done, you will need to restart Softimage before you can use SItoA.
Please refer to the Softimage documentation for other ways to install Softimage add-ons.