If anyone reads through this whole discussion (as I have) about "show in finder" and is still hoping for a solution, I want to add that I found another support site that said Spotlight will find photos in the finder by name. Right click on photo to get the image name and then use spotlight to find the file. If there are multiple files with the same name it shows all of them. Use Show in Finder from Spotlight and voila, there is the file tree too. I just tried this on a newly imported photo and the files are still indexing so it may take some time to find the new photo.
I'm not an "expert" user but a long time one and it is annoying when I have a steep learning curve when upgrading to a new operating system. This Photos change was one of those. Former events from iphoto did not show up in my Albums, however once I played around with copying some of the former events to albums and then deleting them, all of a sudden an album named former events popped up on my album screen and they are just as easy to browse as in iPhoto. Sorting now seems to be done exclusively with Albums.
As a normal user, which apparently Apple engineers are not, I have frequent need to access photos and I have found that exporting them is extremely cumbersome. To upload photos to websites that don't accept drag and drop, you can drag the files themselves to the upload folder. Thumbnails are currently not dragging and dropping pictures from Photos to Wordpress as they did with iPhoto so for now I'm having to find the file name in the library and drag that over.
It would appear there are a few knowledgeable "experts" here and sometimes their advice is don't touch this you will mess it up! Yeah, thanks IT guy. Just like Do It Yourself forums that advise hiring a professional. I confess, I never used Spotlight to find anything, just the find function in the finder. Now I will use it to workaround this issue of not having the "Show in Finder" option in Photos. I may also use "Show Package Contents" and why not give me the benefit of the doubt? If someone is knowledgeable enough to search for answers, they are probably fine with trying to use those answers.
I read about changing to managing the files myself, but that seems like a ridiculous burden just to be able to access an original file. I won't use iPhoto iCloud for my photo library, because I already have to pay for storage just to back up my phone, and I'm not willing to pay for 30,000 images I can store on my own computer hard drive and back up with Time Machine.
I also had to log out of iCloud, restart my iPhone and my computer, and log back into iCloud to get My Photo Stream to download new photos to the new Photos application. Found that advice here though. Spotlight is still indexing--I hope this also works with newly imported photos! I see they are in the same Photo library as the older photos just need Spotlight to get the new files indexed so it can find them. Whew, good luck users!