When you go to the Preferences > VST section, you are supposed to just add the Folder where RitMix.dll is located.
You will not be able to see any files in the folder when adding a Directory to the list in Preferences.
Just add that folder and click the “Find New” button.
After that RitMix should be visible in the Plugin browser.
Just click on VST2 or VST3 and search for RitMix using the search bar on top of the plugin browser.