It seems that this extension is outdated. I did install it once at the time, but now it's turned off and I can still (as @leocg writes) load extensions directly from the Chrome Web Store. On the overview it says if it is compatible with your device and when you install it you get a warning message that the extension is not from Opera addons. It will probably be the case (as it used to be) that Chrome extensions under Opera may not offer all features.