In latest Opera 15, there's no option to use DDG from the address bar.
Agreed. Very annoying. If you could implement DDG from the address bar, that would be wonderful.
I'm sorry to respond this late here but it is now possible to just press "d" and space to search from the address bar if you install the extension in Opera 15+ browser. It's not perfect but it's as much as we could do.
I'll close this issue for now but please feel free to open a new one if you run into any issue.