Not sure, I installed all the CC products first then installed the EXT MGR CC and still have the same results you do. IE I have to open the EXT MGR APP standalone but cant access it within the other products.
I was trying to install GuideGuide and was under the impression that Extension manager would show up in Photoshop. It doesn't. I installed Guide Guide into the Photoshop area in the Extension manager and now Guide Guide shows up in the Photoshop extensions.