Under Preferences / Viewer / Plugins, make sure Raw Digital Camera is enabled, and then click Configure on it. Check that .arw is in its list of extensions.
If that's already set up, is this a .ARW file you've viewed in Opus before, or a new file?
I just got the temporary license so it is ok. However if I open a .arw image in a folder with multiple images I cannot skip to the next image. Preferably I would like to do this with the right arrow button. Can this be done, Leo?