[Cannot open new photos from Mac's Photos app]

Either use the built-in Media Browser (on the View menu) or from the File > Open dialog, scroll down on the sidebar until you see the "Media" category & choose "Photos" indented below it.

 

Either way, you will get choices for Photos, iPhoto, and/or any other Apple app that stores photos in special library containers that Apple does not want you to access directly (because doing that can damage the database & other 'housekeeping' files needed to maintain the library).

That did it! I didn't have split screen view set, and once I did so I found Media category and could move forward. Thanks much!

[Cannot open new photos from Mac's Photos app]

Mac, OX X, El Capitan.

When I File/Open in Affinity, iPhotos (copied from old Mac) and Photos (new work) are both grayed out and inaccessible. This is where all of my photos are; what am I missing or doing wrong? I've been copying from Photos to desktop and Affinity is fine pulling from the desktop, but that is a very short term fix.