PowerPhotos lets you manage multiple Photos libraries, but only one of your libraries can be open in Photos at any given time. In your list of libraries, whichever library has a green checkmark appearing next to it is the active Photos library. Opening Photos, either by using the “Open Photos” button in PowerPhotos, or by clicking on Photos in the Finder or on the dock, will display the active library in Photos.
To switch to another library, just double click the library’s icon in PowerPhotos, and it will take care of quitting Photos, changing the active library, and then reopening Photos to display the library that you double clicked. PowerPhotos keeps a separate copy of your Photos preferences for each of your libraries, and will swap the preferences whenever it switches to a different library. This allows you to have different settings in the Photos preferences window on a per-library basis.
An alternate method is to double click the library itself in the Finder to open that library up in Photos. This works, but using this method will not swap out the Photos preferences like PowerPhotos does when switching libraries.