Why would I have disabled it, since I used it all the time as well in previous versions? The option [u]is[/i] enabled, and the navigation doesn't work unless I select multiple files and push the default Show Pictures in the toolbar. Double-clicking on a single file is not enough. In my book, it is broken -- or it doesn't work the way it used to.
It works fine here. Are you sure it is the Opus viewer that's appearing when you double-click and not the Windows viewer? They look very similar.
Also, we're only talking about double-clicking files in Opus windows, right? The next/prev stuff doesn't currently work if you double-click a file outside of Opus (e.g. on the desktop).