Photos in Mac OS X El Capitan has gotten some solid improvements. My favorite is — no, wait, I'll save that for tomorrow — but I do like that fact that you can now add and edit location information (something iPhotos, the app Photos replaces, was able to do for a long time).
Here's how to geotag a pic in El Capitan's Photos:
Choose the photo you want to tag.
Select Info from the Window menu.
Click Assign a Location in the Info panel that appears and start typing.
That's all there is to it.
OS X El Capitan is available as a free update from the Mac App Store. The operating system upgrade supports all Macs introduced in 2009 and later, and some models introduced in 2007 and 2008.