The ReadMe file included in the download from the first post says:
"Copy the two files OptionBasedLibrary vX.X.dll/.dlc to the Paint.NET\ folder
(not to the Effects folder!)"
Leaving the OptionBasedLibrary in "Documents\paint.net App Files", the program did not add the Menu / Adjustments / Color Matrix option; the Settings / Plugin Errors pane was empty. After reading through this thread, and finding the quoted post above, I moved the library to the Effects subfolder, which fixed this behavior.
I suggest amending the ReadMe in the download to have instructions valid for Paint.net v5+.
