(Hey! I posted an answer and now it is gone? What the hell?)
So here it is again.
In XSI, if you click on the viewport's name, you have in the drop down menu an item called Net View. This is an integrated browser. By default, when you switch to Net View, you are in a database on your machine. This is XSI Local.
But you have another button, called XSI Net. When you click on this, you connect to Softimage's database. On XSI Net you have plenty of scripts, utilities and solutions to known problems.
So the script you want is located in XSI Net > Tools > FILE CONVERTER. It is the first item in the list, Wavefront OBJ Converter 1.3. Click on Download, save the file to your disk. Then, do File > Add-on > Install, and select the .xsiaddon file you just saved on your disk.
Hope this helps
Salutations - Cheers
Bernard Lebel