Adding location data is not supported in Photos. I hope, the next update will add this necessary feature.
Send feedback with a feature request: Apple - Photos - Feedback
For the time being, you have to add GPS coordinates to the photos before importing them to Photos.
You could, for example, first import to iPhoto, add the locations, batch change the titles and captions, do all the things that are not yet supported in the new Photos, then export the photos from iPhoto and import them to Photos.
Or use the free exiftool, if you like the Terminal.
See: http://www.sno.phy.queensu.ca/~phil/exiftool/exiftool_pod.html#geotagging_exampl es
To install exiftool: http://www.sno.phy.queensu.ca/%7Ephil/exiftool/install.html
Other convenient apps are Jetphoto Studio, Geotagalog, houdahGeo, there are many more ..
I use Jetphoto Studio, but it is not free.
If you updated iPhpto to version 9.6.1 before the update, you can still use iPhoto.
Just drag it back to the Dock from the Applications folder.
If you not yet have 9.6.1, update as described here:
Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities
The main idea is to move the iPhoto app to the Trash, so Spotlight does not see it as installed, reload the AppStore, and then to try to download again from the App Store - from your Purchases tab, not from the main page, since you can neither buy iPhoto nor update it, but you can reinstall, if your AppleID is associated with it.