You can download it from the developer's SkyDrive account HERE.
I translated the readme.txt in Google:
It is a plug-in that inputs and outputs Metasequoia format with Softimage mod tool 7.5.
We have not confirmed the operation in Softimage (formerly XSI).
It is confirmed in Windows XP.
· Installation method
Install only "Application" → "Plugins" source file in one of the following locations of Softimage.
· Workgroup C: /Softimage/Softimage_Mod_Tool_7.5
· Add-on Detectors in Work Group C: /Softimage/Softimage_Mod_Tool_7.5/Addons
· User Location C: / users / User Name (Optional) /Autodesk/Softimage_Mod_Tool_7.5
You can not read data containing lines (data consisting of 2 index numbers).
It does not correspond to a curved surface etc.
All mirroring is treated as specular separation.
When outputting, color data is not output properly.
I tested the import and export feature and it works most of the time. It doesn't seem to like importing files with binary vertex data though, for example; the cat.mqo file in the Metasequoia installation, but since the plugin is open source python you could modify it to work how you want.
To export to *.mqo from Mod Tool your mesh objects need to be under a model null.
If it works in Mod Tool then it'll likely work with commercial versions of Softimage also.