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:
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.
Please refer to the Softimage documentation for other ways to install Softimage add-ons.