Just drag and drop the xsiaddon onto the Softimage viewport
Then find the GoSoftLite.zsc in the Softimage Addons/GoSoftimageLiteAddon/zsc folder and copy to the appripriate ZPlugs folder in your ZBrush folder...oh my...
There will be two new menuitem in the File menu. GoSoftLiteExport, and GoSoftLiteImport. As the names imply the Export will export the current geometry, while the import will replace the current geometry. If you don't want to replace your object, just deselect it prior the import.
In Zbrush there will be a new subpalette at the end of the Tool palette, GoSoftimage Lite, with two button, GoSoftimageExp, and GoSoftimageImp. It will export/import the current subtool at the current level.
Basic operation is that the two packages communicate via an obj file (c:\GoSoftLite.OBJ").
Geometry, UV exported, imported pretty well
ZB textures exported to Softimage, but not yet flipped