The original download links are not working at the moment. For the moment, you can download the plugin from:
DOpus UGV Plugin for Opus 32-bit 0.9.0.5 636 KB
DOpus UGV Plugin for Opus 64-bit 0.9.0.5 1.37 MB
To install, just extract the contents of the ZIP archive to the Directory Opus program folder (usually C:\Program Files\GPSoftware\Directory Opus).