How to add a location to photos in the new photos app?
In OS X 10.10's Photos application, location information is read-only.

Photos reads GPS coordinates from image files during import and displays the location in the Info window. It can also show photos belonging to a collection or moment on a map. The "Photos Help" puts it this way: "If your camera has GPS capability (as iPhone does), or you’ve added GPS information to your photos, Photos can automatically display your photos on a map."

Unfortunately location information is also read-only by the way of Apple Script. I develop the photo geocoding software HoudahGeo. When used with iPhoto or Aperture, this allows for a very elegant workflow:


Import photos into iPhoto or Aperture 
Geocode the photos using HoudahGeo or other free software.
Write geotags to the original image files
Have HoudahGeo notify iPhoto or Aperture of the newly added geotags


The last step of the workflow uses Apple Script to update the library to reflect the location information added to the files. This is not currently possible with the new Photos application.

Until Apple adds the option to change location information via Apple Script, we have to resort to a less elegant workflow.


Import photos using the Image Capture application 
Geocode the photos using third party software like HoudahGeo or other free software.
Write EXIF/XMP geotags to the image files 
Import the tagged files into your Photos library


This workflow does have its advantages. The process of updating the iPhoto or Aperture library for thousands of photos can be rather slow. Adding the GPS information before import is faster.

The above workflow also ensures that GPS information is stored as EXIF/XMP tags within the image files. Just the same as if you had used a GPS camera. The location information will thus always stay with the images.

Update for macOS Sierra:

With Apple Photos 2.0 and HoudahGeo 5.1 running on Sierra it again possible to notify Photos of changes made to location information. I.e. HoudahGeo will geotag orginal image files with EXIF/XMP tags and then pass on location information to the Photos app.