You cannot unpack MUI with UnArc if you didn't install Boingbag1 first. UnArc of OS 3.9 witrhout Boingbags cannot handle files with spaces in their names correctly. The MUI archive contains one file whose name only consists of spaces. This file is needed for installation.
The guide you linked above mentions the workaround for this error in the Troubleshooting section at the end of the page.