That message means that the installer could not look-up the Opus directory in the registry.
You can either manually add the registry setting or install the plug-in by hand. Fixing the registry is a bit more effort but might save time in the future. I'll describe both things and leave it up to you to decide which to do depending on your familiarity with registry editing:
[ul][li]Registry setting:
Open RegEdit.exe and navigate to here:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths
If there isn't already, create a DOpus.exe sub-key.
Within that key should be two REG_SZ values. The default (nameless) value should be:
C:\Program Files\GPSoftware\Directory Opus\DOpus.exe
There should also be a Path value set to:
C:\Program Files\GPSoftware\Directory Opus
(Of course, modify those if you've installed Opus somewhere else.)
[/li]
[li]Alternatively, install the plugin by hand:
Grab the zip of the plugin, instead of the .exe installer, extract it somewhere.
Exit Opus. (See here.)
Using Explorer (or something other than Opus), copy the files you extracted into your Viewers directory. The directory will usually be here:
C:\Program Files\GPSoftware\Directory Opus\Viewers
Restart Opus.
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