PowerPhotos 1.0.6 for OS X Yosemite
Photos on OS X Yosemite had some critical bugs that prevented PowerPhotos from being able to reliably implement its photo copying and library merging features. These bugs were fixed by Apple in the next update, El Capitan, so the current version of PowerPhotos requires that you be running El Capitan or later.
However, if you're still running Yosemite and can't or don't want to upgrade to El Capitan yet, you can download PowerPhotos 1.0.6, which still runs on Yosemite. This older version isn't able to copy photos or merge libraries, but you can still use it to manage your libraries and search them for duplicate photos.